__—_ —L

i()noui _ /_brain

nerve cord pjoipbiou


- ip"^ uiejq mouth

Fig. 8. Diagram to illustrate the transformation of an annelid into a vertebrate, as envisaged by Anton Dohrn and others. Redrawn with modifications from Fig. 182 in Wilder (1923).

Fig. 8. Diagram to illustrate the transformation of an annelid into a vertebrate, as envisaged by Anton Dohrn and others. Redrawn with modifications from Fig. 182 in Wilder (1923).

originally proposed by Geoffroy-St. Hilaire (1822, 1830). This idea is well put by Semper (1875): "If the embryo of the annelide be turned so that its ventral surface lies upward, its section presents exactly the same arrangement of the organs as the Selachian embryo. Consequently, by the discovery of the segmental organs, the belly of the annulose animal is identified with the back of the vertebrate" (details in Wilder (1923), pp. 570-80).

CD had earlier encountered a version of this: There is no scale, according to importance of divisions in arrangement, of the perfection of their separation. - thus Vertebrate blend with Annelida by some fish (Notebook D, p. 370). However, CD, who in a letter to Dohrn expressed that the work astonished him (see Calendar, no. 9991) did not even mention it when later discussing *vertebrate origins. Having identified the larvae of ascidians (or *seasquirts) as progenitors, he did not bother much with their ancestry.

And yet, the recent discovery that similar homoeotic or *Hox genes, in a wide range of vertebrates and invertebrates, determine their basic body plans and the relative location of their organ systems, is compatible with the suggestion that the vertebrates are derived from flipped-over, bilaterally symmetrical animals also ancestral to annelids, the Ur-bilateria of De Robertis and Sasay (1996). [I added the hyphen, lest some read the new name as Urbi-lateria, i.e. urban latecomers in Very Low Latin].

Gould (1998a, p. 334), when presenting the related story of Gaskell's (1908) derivation of vertebrates from arthropods, suggested that, while supporting Geoffroy-St. Hilaire's "old theory of inversion", the presently available data "do not support the silly notion that at some defining point in the march of evolutionary *progress, an arthropod literally flipped over to become the first vertebrate". This is rhetorical overkill: yes, no arthropod ever turned into a vertebrate, by flipping over or otherwise, and whether marching for progress or not. However, the idea that animals may develop the habit of'flipping over', then, over evolutionary time, acquire anatomical features reflecting this flip is not "silly" at all: flatfishes do it all the time, and CD used unflappable logic to explain how this could happen. (See also Flatfish controversy.)

And while it can be argued that flatfishes have flipped by only 90°, I can think of several fish species that have flipped by 180°, and do not seem the worse for it. One example is the Blotched upside-down catfish Synodon-tis nigriventris David, 1936, whose common and scientific names (nigriventris = blackbelly) say it all. [There is also my small but instructive contribution to the Annals of Improbable Research (Pauly 1995a), which deals with upside-down fishes, and which is silly.]

The neat part about this whole thing, as noted by Kinsbourne (1978), is that it provides a testable hypothesis to explain why vertebrates have the left side of their *brain connected to the right side of the body and vice versa. In our ur-bilaterian ancestors, the *eyes (they did have eyes; De Robertis and Sasay 1996) and their associated image processing cells (the original brain) may have experienced a first torsion when their bodies became reoriented by 90°, from vertical to horizontal (as in the *ontogeny of flatfishes). Then, some populations may have returned to a vertical posture, but not by reversing the previous torsion (as

Dredging seem to be presently occurring with *Green-land halibut), but by completing the 180° turn (see also Pinker (1994), pp. 303-5). This would allow interpreting the partial asymmetry of the *lancelet (Gee 1994), one of the simplest chordates, as a vestige of a rotation, with *Natural selection not having come around to re-establishing the original, full bilateral symmetry of the unflipped progenitors.

Dragonets Members of the family Callionymi-dae, one of the few taxa of colourful fishes in temperate waters: There is reason to suspect that many tropical fishes differ sexually in colour and structure; and there are some striking cases with our British fishes. The male Callionymus lyra has been called the gemmeous dragonet 'from its brilliant gem-like colours.' When fresh caught from the sea the body is yellow of various shades, striped and spotted with vivid blue on the head; the dorsal fins are pale brown with dark longitudinal bands; the ventral, *caudal, and anal fins being bluish-black. The female, or sordid dragonet, was considered by *Linnaeus, and by many subsequent naturalists, as a distinct species; it is of a dingy reddish-brown, with the dorsal fin brown and the other fins white. The sexes differ also in the proportional size of the head and mouth, and in the position of the eyes;12 but the most striking difference is the extraordinary elongation in the male ([Fig. 9]) of the dorsal fin. Mr. W. Saville Kent remarks that this 'singular appendage appears from my observations of the species in confinement, to be subservient to the same end as the wattles, crests, and other *abnormal adjuncts of the male in gallinaceous birds, for the purpose of fascinating their mates.'13 The young males resemble the adult females in structure and colour. Throughout the genus Callionymus,14 the males is generally much more brightly spotted than the females, and in several species, not only the dorsal, but the anal fin is much elongated in the males. (Descent


Fig. 9. Dragonet Callionymus lyra, Family Callionymidae, illustrating sexual dimorphism, a result of sexual selection. From p. 337 in Descent II, based on a woodcut by Mr G. Ford, prepared under the guidance of Dr. Giinther. A Female: B Male.

II, pp. 335-6; n. 12 reads: I have drawn up this description from Yarrell's British Fishes, vol. I, 1836, pp. 261 and 266.; n. 13 cites Saville-Kent (1873b); n. 14 cites Giinther (1861), pp. 138-51).

Linnaeus (1758,p. 249)indeed described two species ofdragonet from Europe (in addition to one from India, now assigned to another family). One was Callionymus lyra, the other C. dra-cunculus (i.e. 'small dragon', or 'dragonet'). C. dracunculus is now a 'suppressed' name, synonymous with C. lyra. Dredging An important research method, adapted from a commercial fishing gear, and used to obtain sampling marine sediments, and the *zoobenthos (Rehbock 1979). CD reports in his *Autobiography on dredging trips he undertook when he was a student in *Edinburgh: I also became friends with some Newhaven fishermen, and sometimes accompanied them when they trawled for oysters, and thus got many specimens. (p. 50).

The samples obtained by systematic dredging surveys, notably by Edward Forbes and Karl Mobius, led to numerous, and much debated, generalizations about the distribution of

Fig. 9. Dragonet Callionymus lyra, Family Callionymidae, illustrating sexual dimorphism, a result of sexual selection. From p. 337 in Descent II, based on a woodcut by Mr G. Ford, prepared under the guidance of Dr. Giinther. A Female: B Male.

Dules spp.

*zoobenthos along depth gradients, and its tendency to form recurrent communities, respectively (Rumohr 1990). Rehbock (1979) provides an entry point into this literature, which does not concern fish directly, as dredges, contrary to trawls, do not usually capture fish and other agile animals.

Driftfishes Members of the Family Nomeidae. Two specimens taken on March 23, 1832 (Zoology Notes, p. 325) by CD at 17°12'S and 36°33'W (i.e. northeast of the *Abrolhos, Brazil) were tentatively attributed by Jenyns to Psenes leu-curus (Fish, pp. 73-4), i.e. to what is now the Freckled driftfish Psenes cyanophrys Valenciennes, 1833.

Duckbills Members of the flatheaded Family Percophidae, which includes the Brazilian flathead Percophis brasilianus Quoy & Gaimard, 1825, described in Fish (p. 23), based on a specimen collected by CD in northern *Patagonia.

This fish is known in Brazil to be "de excelente gosto" (Carvalho-Filho 1994, p. 196), in line with CD, who noted: When cooked, was good eating. (FishinSpirits,nos. 347,692;seealso Oxford University Museum.)

Dules spp. See Groupers (I).

Ecology A term coined by *Haeckel to refer to the scientific discipline devoted to studying the relationships of organisms to their environment. More precisely, autecology refers to single species and their abiotic and biotic environments, while synecology deals with interrelationships among species.

Ecology is a relatively new discipline, at least compared with other branches of biology. This is understandable, as it was first necessary to describe and name a representative part of the Earth's species (see Taxonomy), and to understand their basic functional anatomy and physiology before their ecology could be understood.

This is well illustrated by CD's failure to apprehend the structure of pelagic *food webs, owing to his misunderstanding the nature of *plankton.

Still, CD was a better ecologist than most biologists of his time (see Stauffer 1960): the theory of *natural selection required plausible scenarios for the modification of all features of animals and plants (morphological, physiological, behavioural...) and this required knowledge of how they relate to their environment, and to each other.

Edinburgh A city in Scotland, home of the University at which young CD started the study of medicine in October 1825, abandoned in April 1827 (Ashworth 1935).

The many connections of the Darwin family to Edinburgh are reviewed in Shepperson (1961), along with the intellectual history that led to Scotland's 'Golden Age', crucial to the development of CD's ideas.

CD's first scientific writing - an account of his dissection of a *Lumpfish, performed under the guidance of his mentor, Prof. R. Grant - was written in Edinburgh, as was his subsequent, better-known discovery that the so-called ova of Flustra had the power of independent movement by means ofcilia, and were in fact larvœ. (Autobiography, p. 50; Flustra is a Bryozoan, one of CD's Zoophites.)

Edmonston, John A taxidermist based in Aberdeen, from whom, in 1826,young CD learnt the basics of preparing animal specimens (Corresp. to sister Susan Darwin, Jan. 29,1826).

What little information is available on Edmonston is summarized in a patronizingly titled contribution by Freeman (1978/79). Edmonston appears to have contributed much to CD's positive perception of people of African ancestry, and to his rejection of slavery, widely accepted by his English contemporaries.

Taxidermy was important in CD's time because lack of suitable, tight containers made it difficult to keep specimens in alcohol and other liquids. Hence, not only bird and mammal specimens were reduced to dried skins, but also fishes (see also Collection; Galapagos).

Eel The 'Common' or European eel Anguilla anguilla (Linnaeus, 1758), which enters river mouths as small transparent 'glass eels', grows, then returns to the sea to spawn (see also Eels).

CD wisely abstained from suggesting a precise spawning location for adult eel, then still a popular guessing game, and one which brought shame to virtually all practitioners, starting with Aristotle (see Aristotle 1962), who had them spontaneously generated by "earth guts that grow spontaneously in mud and in humid ground" (Historia Animalium, Book VI, 15). Indeed, this issue was resolved only through the 1922 expedition of the Danish scientist Johannes Schmidt to the Sargasso Sea, in the mid-Atlantic (Nikol'skii 1961, pp. 315-16; Muus and Dahlstr0m 1974, pp. 82-3), where eel now spawn. When the Atlantic was young and narrow, both European and American eel probably spawned on the shelf edge. As Europe and America drifted apart, and the Atlantic widened (Wegener 1966), these eel found themselves having to swim every year a few centimetres further offshore to reach the same spawning grounds. This, however, is the very mix of change and obstinacy leading, via millennia

Eels of *natural selection, to adaptations that look miraculous to those not firmly rooted in the natural sciences (see also Distribution; Obstinate nature).

On the other hand, CD, whose work is so neatly vindicated by the reproduction of Eel, mentions them in one ofhis many discussions of the evolution of *eyes. Here, he cites from an account in some French Transactions (Correspondence to J. M. Rodwell, Nov. 5, 1860), as follows: it has been 'remarked that fishes which habitually descend to great depths in the ocean have large eyes'.1 And one most remarkable fact is on record, <which is worth giving, though of a most perplexing nature. > M. Eudes-Deslongchamps gives with great detail two cases2 of eels taken from wells about 100 feet in depth, which had their eyes of immense size, so that their upper jaw in consequence projected over the lower. (Big Species Book, pp. 296-7; n. 1 refers to Richardson (1856), p. 219; n. 2 to Eudes-Deslongchamps (l835,1842)).

But here comes the remarkable fact the first specimen was shown to *Agassiz, & he thought it was specifically identical with the common Eel. One of the wells was within the precincts ofa prison; & it seems impossible to conjecture how the eel got in; & it seems, moreover, quite incredible that such an alteration could have supervened during one generation: it is, also, most improbable that there should be a *race of subterranean eels, for, I believe it is well established that the eel invariably breeds in the sea.

Surrounded with difficulty as this case is, we apparently have in the large eyes of these eels, & in the blind Gadus from the deep parts of the lake Leman, a parallel case to the opposite condition of the eyes of the Kentucky *cave-fish... (BigSpecies Book,p. 297; see Burbot for the Gadus of Lake Leman).

The context ofthis story is provided in a letter to C. *Lyell: very important characters may be modified by correlation of growth. But I

doubt whether they throw light on abrupt origin of new forms. [. . .]. With respect to animals, besides the case of monstrous *Gold-fish with analogous fish in state of nature alluded to, I have wondrous case of monstrous eels, (examined by Agassiz) & apparently produced by darkness, but I cannot satisfy myself on case; nor does it appear certain that they breed. (Correspondence, Feb. 23, 1860; 'correlation' is here meant as the normal coincidence of one phenomenon, character, etc., with another; Origin yi,p.432).

In Northern Europe, two *varieties of eel are often alleged to exist, one pointy-, the other broad-headed. Thurow (1953) demonstrated that these represent the extremes of a wide range of head shapes, mostly due to different diets: pointy-headed eels tend to feed on worms and small crustaceans, broad-headed ones on larger crustaceans and fishes.

This plasticity of head shape makes it seem quite possible for our imprisoned eels to have developed large eyes in the course of their *ontogeny. Eudes-Deslongchamps (1835) stresses how "impossible" it was for these macrophthalmic eels to have reached their well from any other water body, and that nobody ever gave eels to the prisoners.

But then, nobody gave birds to the Birdman ofAlcatraz, and still he had a lot ofthem.

Eels A group of elongated fishes belonging to the Family Anguillidae, and allied groups, such as the Congridae (conger eels).

When referring to 'eel' CD generally means the European, or 'Common' *eel; e.g. in the quote Professor Ercolani has recently shown (Accad. Delle Scienze, Bologna, 28 December, 1871) that eels are *androgynous (Descent I, p. 161, n. 28; incidentally, Professor Ercolani (1871a, b) was quite wrong).

However, other eels show up, for example in *Tahiti, where CD, describing an improvised meal taken after the ascent of a steep, narrow valley, mentioned that a little stream, besides

Eggs of fish its cool water, produced eels and cray-fish. (Journal, Nov. 18,1835).

There are three species of eel in Tahitian inland waters (Marquet 1992; Marquet and Galzin 1991,1992): Anguilla marmorata Quoy & Gaimard, 1824, occurring mainly in running waters, below waterfalls; A. megastoma Kaup, 1856, in running waters, above waterfalls, and A. obscura Gunther, 1872, in estuaries and shallow stagnant waters. Our best candidate for the species that contributed to CD's meal is thus A. megastoma.

Another species is the Shortfin eel Anguilla australis Richardson, 1841, sampled by CD in late December 1835 at the *Bay of Islands, New Zealand (Fish in Spirits, no. 1337), and whose colour *Jenyns (1842, p. 142) described as "similar to that of the common *eel".

(See also Heron, Otters, Morays; Fish in Spir-its,no. 431).

Eelpouts Fishes of the Family Zoarcidae, not related to the eel(s), and represented here by three species.

The first is Phucocoetes latitans Jenyns, 1842, caught amongst kelp in the *Falk-land Islands (Fish in Spirits, nos. 598-599; Fig. 10A).

The second, Iluocoetesfimbriatus Jenyns, 1842, is described in Fish, based on a specimen from

Fig. 10. Eelpouts, Family Zoarcidae, with inserts showing teeth (Fish, Plate XXIX). A Phucocoetes latitans, from kelp beds in the Falkland Islands. B Iluocoetesfimbriatus from Chiloe Island, Chile.

*Chiloe, Chile (pp. 166-7). CD noted on this Blennius under stones (Fish in Spirits, no. 1080; Fig. 10B), mistaking his find for one of the *blennies.

The third, based on a small specimen with Sides with transverse bars of chocolate and brownish-red, separated by narrow grey spaces was originally assigned to Conger, a genus of *eels (Fish, p. 143; Fish in Spirits, nos. 866,870). It is now called Mayneapuncta (Jenyns, 1842).

Another member of the Zoarcidae is the European species Zoarces viviparus (Jenyns, 1758), whose German common *name (Aalmutter, meaning 'Mother of eels' and referring to their giving birth to complete, eely-looking young) shows that one must be careful with folk etymologies (see Names, common), which can be as misleading as scientific names.

Eggs of fish CD clearly felt that the large number of eggs in fish supported his theory of *natural selection, as it suggested - given habitat limitations - extremely high mortalities for both the eggs and the larvae hatching out of those eggs.

Thus, struggle & destruction follows inevitably in accordance with the law of increase so philosophically enunciated by Malthus1 (Big Species Book, p. 176; Malthus 1826; n. 1 states Franklin & many others have clearly seen & exemplified the great tendency to increase in all the lower animals and plants; Franklin 1755).

CD was right (whatever one might personally feel about Malthus), and here is one of his few quantitative comments on this: Everyone must have seen statements of the number of eggs & seeds produced by many of the lower animals and plants.2 To illustrate geometric progression one meets in works on arithmetic calculations such as, that a Herring in eight generations, each fish laying 2000 eggs, would cover like a sheet the whole globe, land & water (Big Species Book, pp. 176-7; n. 2 reads as follows: I will copy out a few instances of numbers of eggs & seed. Mr. Harmer in Phil.

Egg development

Transact. 1767, p. 280 weighed the whole & portions ofroe and counted in this portion the number of eggs. The numbers differed considerably in different individuals

*Carp 203,109 and 101,200 lowest number

*Cod 3,681,760

*Flounder 1,357,400 and 133,407 do

*Herring 36,960 and21,285 do

*Smelt 38,272 and 14,411 do

(N.B. These observations [. . .] are confirmed by independent calculations by C.F. Lund in Acts of Swedish Academy Vol. 4). [For Smelt, the lower number in Harmer (1767), p. 285 is 38,278; somebody made an error transcribing that number, but it was not necessarily CD.]

Clearly, high fecundity (and the high mortality that consequently ensues) implies an enormous role for predation, as recently restated for fishes by Christensen (1996).

Another twist to the issue of high fecundity is that it does not preclude rarity, or in CD's words How many rare fish there are existing in very scanty numbers, yet annually producing thousands of ova! (Big Species Book,p. 206; see also Water-beetles).

Egg development CD expressed much interest in the relationship between the development and survival of seeds and ova (see Experiments), notably fish eggs, as this topic closely relates to the issues of the geographic distribution of plants and animals.

One item on this, based on Stark(1838), is roe ofAsterias in stomach ofsammon remain after rest of animal digested (Notebook C, p. 267; and yes, the *spelling of*salmon is not correct, but what a great way to show how this English word should be pronounced!)

The most explicit comments on this issue are found, however, in CD's response to a letter of March 21, 1855 from John Davy, in which were described results of experiments later published in Davy (1856). It was shown therein that the survival of fertilized salmon eggs depends not only on their being kept moist, but also on temperature, and on their developmental stage.

To this CD commented: The case of the ovum exposed on the moss for three days, & the wonderful retention of vitality of the ova in very moist air seem of particular value in regard to the power of dispersal. Surely these results will, also, be of practical value.

Hardly anything has surprised me more than the non-developed ova having less tenacity of life than those much more fully developed. -I almost hope that shd you ever have another opportunity, it may be worth while to test this one point again.

I have been much struck by your experiments on the effects of rather high temperature; I have often speculated whether the ova accidentally introduced into the stomach of an herbivorous bird could escape the action of the gastric juices, but I had not at all calculated on the very injurious action of the mere temperature.

With many such experiments as yours, Geographical *Distribution would become in my opinion, a very different subject to what it is now... (Correspondence, March 26,1855).

Later, CD let Hooker know how pleased he was about Davy's results: My notions sometimes bring good; Dr. Davy has been experimenting at my request, (in order to see how fishes' ova might get transported) on the retention of vitality; & he found that salmon's ova, exposed for 3 whole days to open air, & even some sun-shine, & they produced fine young fish. Dr. Davy has sent a paper to Royal Soc. on the subject. - N.B. Remember to ask about my distinct case of'a lady in N. America' who saw fishes' spawn adhering to a Ditiscus (Correspondence, April 7, 1855; for the lady in N. America see Morris, Margaretta Hare; for Ditiscus, see Water-Beetles).

Electric fishes

Electric eel Common name of the fish which Linnaeus (1758) described as Gymnotus electri-cus, but later renamed Electrophorus electricus (Linnaeus, 1766). This fish, which can reach lengths of well above 2 m, and occurs in deep rivers ofthe Orinoco and Amazon, is feared for its ability to inflict powerful electric jolts. But scientists have loved it since 1729,when Richer, an astronomer, first reported to the French Academy on its electric abilities (Moller 1995, pp. 15 and following). For CD, the Gymnotus, as he called it, was a big challenge, as it was difficult, in his time, to conceive of transitory stages for the *analogous organs of this and other *electric fishes.

CD mentions a possible case of 'inherited memory' in electric eel in a letter (not seen) sent to G. J. Romanes on Jan. 24, 1877 or 78 (Calendar, no. 10813).

Electric fishes Common name for a taxonomi-cally heterogeneous group, all capable of generating electric fields used to detect, and/or stun, potential prey and predators.

As suggested in Moller (1995), fishes can be arranged, in terms of their electrical abilities into four groups: (1) No special ability beyond that ofgenerating a weak electric field when their muscles contract, as all animals do. (2) Electrosensing only, i.e. the ability to sense the electric fields generated by other animals. Electrosensing is the rule in *sharks, *rays, and *chimaeras, i.e. 470 species in 42 families and 14 orders. (3) Weakly discharging: the ability to generate a relatively weak electric field, used mainly for orientation when visibility is low, and for prey detection. The *elephant-fishes are an example of this group, consisting of 407 species in 11 families and 5 orders. (Note that this ability implies electrosensing, as well.) (4) Strongly discharging: the ability to generate strong electric fields, and to stun potential prey and predators, and the only ability of which CD was fully aware. This ability, which involves 38 species in 5 families and 4 orders, implies electrosensing as well (e.g. in *Gymno-tus,or*Torpedo), except in the stargazers, Family Uranoscopidae, which stun without sense.

CD was worried by the implications, for his theory of *natural selection, of the existence of *electric organs in fish, because he thought they were [c]ases of organs in which there is no apparent passage or transition from other organ: or still better, if such transition can be shown in an unexpected manner. E.G. Electrical organs in Fish, seem to be really new organ & not any other changed. (Correspondence to *Huxley, Dec. 13, 1856). At the time this letter was written, CD was working on his Big Species Book, in which he elaborated upon this theme: The electric organs ofFish, - those wonderful organs which, as *Owen says, 'wield at will the artillery of the skies' - offer a special difficulty. Their intimate structure is closely similar to that of muscle;3 but it is most difficult to imagine by what grades they could have arrived at their present state.4 Nevertheless the fact, <recently discovered> that Rays5 which have never been observed to discharge the feeblest shock, yet have organs closely similar to those of true electric fishes, shows that we are at present too ignorant to speculate on the stages by which these organs, now affording such a powerful means of defence to the *Tor-pedo & *Gymnotus, may have been acquired. But the special difficulty in this case lies in the fact that the Electric fishes, only about a dozen in number, belong to two or three of the most distinct orders or better sub-classes of Fish.6 (BigSpecies Book, p. 363; n. 3 cites Owen (1846), p. 217; n. 4 reads Dr. Carpenter [1854] in his Principles of Comparative Physiology (iv. Edit.) has an interesting discussion on the Electric organs of fishes: compare p. 465-470, & 471; n. 5 cites Stark (1844) and adds on Jan 6,1845 Mr. Goodsir read paper on same subject, & shows that the organ in the Ray is the middle & posterior of the *caudal muscle, greatly modified; see Goodsir (1855); n. 6 cites Valenciennes

(1841), p. 44; CD underestimated the number ofelectric fish species).

CD was so charged up with electric fishes that he discussed them with George Henry Lewes, "a man of letters" known mainly for having been the common-law husband of Mary Ann Evans, a.k.a. George Eliot (Freeman 1978, p. 188).

Thus, in a letter titled Against organs having been formed by direct action of medium CD writes to him that If you mean that in distinct animals, parts or organs, such for instance as the luminous organs of insects or the electric organs offishes, are wholly the result ofthe external and internal conditions to which the organs have been subjected, in so inevitable a manner that they could be developed whether of use or not to their possessor, I cannot admit [your view.] Moreover, CD notes the strange implications of Lewes's notions: [if] I should apply the same doctrine to the electric organ of fishes [I would] have to make, in my own mind, the violent assumption that some ancient fish was slightly electrical without having any special organ for the purpose (Darwin and Seward 1903, Vol. I, pp. 306-7).

In any case, CD need not have worried. Roberts (2000), in his review of the Malapteruridae, a family of strongly discharging catfishes, points out that CD "failed to realize that electricity could modify the behaviour of small prey, either by immobilizing it or causing it to react in such a way that it would be more susceptible to predation. The initial behaviour leading to *natural selection favouring weak electric organs in fishes, is surely more efficient feeding on small organisms, such as tiny vermiform aquatic insect larvae. From this, it would be only a short step to weakly electrogenic conspecifics joining each other in foraging activities and the simplest form ofelectrocommunication (detecting weak electrical discharges signalling that conspecifics are engaged in feeding behaviour). And from this it seems a short step to the development of electrical signalling related to reproductive behaviour, and perhaps the most derived function of all, electro-location. There would be many opportunities for occurrence of the phenomenon known as 'intensification of function' leading to the gradual improvement of the electrogenic and electrosensory organs by means ofnatural selection and perhaps also *sexual selection."

Electric organs (I) The electric organs of fishes are given much emphasis in the first edition of Origin (1859) where they are discussed in the chapter titled *Difficulties on theory, and to which CD attached much importance, for example advising his doubting friend C. *Lyell: Please read p. 193 beginning 'The Electric organs' & trust me that the sentence 'In all these cases of two very distinct species &c &c' was not put in rashly; for I went carefully into every case (Correspondence, Sept. 23,1860).

We shall follow the advice: The electric organs of fishes offer another case of special difficulty; it is impossible to conceive by what steps these wondrous organs have been produced; but, as *Owen and others have remarked, their intimate structure closely resembles that of common muscle; and as it has lately been shown that *Rays have an organ closely analogous to the electric apparatus, and yet do not, as Matteuchi asserts, discharge any electricity, we must own that we are far too ignorant to argue that no transition of any kind is possible.

The electric organs offer another and even more serious difficulty; for they occur in only about a dozen fishes, of which several are widely remote in their affinities. Generally when the same organ appears in several members of the same class, especially if in members having very different habits of life, we may attribute its presence to inheritance from a common ancestor; and its absence in some of the members to its loss through disuse or Natural selection. But if the electric organs have been inherited from one ancient progenitor thus provided, we might have expected that all electric fishes would have been specially related to each other. Nor does *geology at all lead to the belief that formerly most fishes had electric organs, which most of their modified descendants have lost. (Origin I, p. 193; see also Matteucci (1843,1847)).

Electric organs (II) In the sixth, and last edition of Origin revised by CD, some of the *difficul-ties for the theory due to electric organs were resolved, while others had emerged. Thus, we can use the revised text to provide a typical example of how CD incorporated new information and stylistic improvements into subsequent editions of his books (added words are underlined, deleted words are crossed through). This is messy, so bear with us:

The electric organs of fishes offer another case of special difficulty; for it is impossible to conceive by what steps these wondrous organs have been produced; but, as Owen and others have remarked, their intimate structure closely resembles that of common muscle; and as it has lately been shown that Rays have an organ closely analogous to the electric apparatus, and yet do not, as Matteuchi asserts, discharge any electricity, we must own that we are far too ignorant to argue that no transition of any kind is possible. But this is not surprising, for we do not even know of what use they are. In the *Gymnotus and *Torpedo they no doubt serve as powerful means of defence, and perhaps for securing prey; yet in the Ray, as observed by Matteucci, an analogous organ in the tail manifests but little electricity; even when the animal is greatly irritated; so little, that it can hardly be of any use for the above purposes. Moreover, in the Ray, besides the organ just referred to, there is, as Dr. R. M'Donnell has shown, another organ near the head, not known to be electrical, but which appears to be the real homologue of the electric battery in the Torpedo. It is generally admitted that there exists between these organs and ordinary muscle a close analogy, in intimate structure, in the distribution of the nerves, and in the manner in which they are acted on by various reagents. It should, also, be especially observed that muscular contraction is accompanied by an electrical discharge; and, as DrRadcliffe insists, 'in the electrical apparatus of the torpedo during rest, there would seem to be a change in every respect like that which is met with in muscle and nerve during rest, and the discharge of the torpedo, instead of being peculiar, may be only another form of the discharge which attends upon the action of muscle and motor nerve.' Beyond this we cannot at present go in the way of explanation; but as we know so little about the uses of these organs, and as we know nothing about the habits and structure of the progenitors of the existing electric fishes, it would be extremely bold to maintain that no serviceable transitions are possible by which these organs might have been gradually developed.

The electric These organs appear at first to offer another and far more serious difficulty; for they occur in only about a dozen kinds of fishes, of which several are widely remote in their affinities. Generally When the same organ appears is found in several members of the same class, especially if in members having very different habits of life, we may generally attribute its presence to inheritance from a common ancestor; and its absence in some of the members tostrikethroughloss through disuse or Natural Selection. But So that, if the electric organs hadbe inherited from some one ancient progenitor thus provided, we might have expected that all electric fishes would have been specially related to each other; but this is far from the case. Nor does geology at all lead to the belief that formerly most fishes formerly had possessed electric organs, which most of their modified descendants have now lost. But

Embryology when we look at the subject more closely, we find in the several fishes provided with electric organs, that these are situated in different parts of the body - that they differ in construction, as in the arrangement of the plates, and, according to Pacini, in the process or means by which the electricity is excited - and lastly, in being supplied with nerves proceeding from different sources, and this is perhaps the most important of all the differences. Hence in the several fishes furnished with electric organs, these cannot be considered as homologous, but only as analogous in function. Consequently there is no reason to suppose that they have been inherited from a common progenitor; for had this been the case they would have closely resembled each other in all respects. Thus the difficulty of an organ, apparently the same, arising in several remotely allied species, disappears, leaving only the lesser yet still great difficulty; namely, by what graduated steps these organs have been developed in each separate group of fishes. (Origin VI, pp. 150-1; Radcliffe 1871; Pacini 1853).

Readers interested in more detailed changes from the first to the sixth editions, and in the intermediate versions of Origin, should consult Peckham's (1959) variorum edition.

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