Diacope marginata See Snappers

Diadromy(-ous) Referring to regular, physiologically mediated migrations of certain stages of fish between fresh waters and the sea, and involving the majority of the members of a population (Myers 1949; McDowall 1988,1997).

Diadromy has two basic forms, anadromy (as in *salmon), where spawning occurs in rivers, while the adults feed and grow in the sea, and catadromy (as in *eels), where spawning occurs in marine waters, and the adult stages feed and grow in rivers, lakes, and other freshwater bodies.

A third form of diadromy, 'amphidromy', has been proposed by Myers (1949), wherein larval fish migrate to the sea after hatching and return to fresh waters as juveniles, with spawning and post-juvenile growth thus occurring in fresh water. However, clear-cut cases seem hard to find (some *Galaxiidae?). McDowall (1997) feels that amphidromous fish are not truly diadromous because they spawn and perform most of their growth in the same medium.

One important aspect of diadromy is that it tends to be connected with *homing, especially so in *salmon. So far, however, only one author, McDowall (2001), has publicly wondered why it should be so. Perhaps the answer is that all fish 'home' (see Obstinate Nature), in which case the occurrence of diadromy is all there is to explain. And this may not be too difficult given the latitudinal comparisons ofGross et al. (1988), who showed that marine waters, at high latitudes, tend to be more productive than rivers and lakes, and conversely at low latitudes. Hence the preponderance of salmon-type anadromy at high latitudes, and of eel-type catadromy at low latitudes.

Diary Short title of Charles Darwin's Diary of the Voyage of H.M.S. Beagle edited by CD's granddaughter Nora Barlow (1933).

There is also a version of the Diary edited by Richard Darwin Keynes (1988), another of CD's descendants.

The Diary provided several of the entries in this book. However, its references to fishes were ignored if they overlapped substantially with the corresponding accounts in CD's *Journal, which was largely adapted from the Diary and is more widely available.

Difficulties CD compiled, in a courageous chapter titled Difficulties on theory (*OriginI, pp. 171206) or Difficulties of the theory (*Origin VI, pp. 133-67), those features of organisms which he himself (or others) had identified as hard to explain through his theory of *natural selection. Most ofthese 'difficulties' were more apparent than real; for example, the alleged 'perfection' of the vertebrate *eye, and the alleged lack of a selection advantage for animals possessing 'imperfect eyes' over animals lacking eyes altogether, have been conclusively shown to be red herrings (Dawkins 1996, pp. 126-79), though they continue to be bandied about (Behe 1996).

In *Foundations, the manuscripts that anticipated Origin, CD included a section on Difficulties in the acquirement by Selection of complex corporeal structures, dealing, as may be expected, with the existence, in various organisms, ofintermediate behaviours or structures, not yet 'perfect' but already conferring a selective advantage on their possessors. Examples

Distribution are the shocking habits of *electric fishes, the aquatic habits of the *jaguar, and the *gasblad-der, or the eyes of fishes: In considering the eye of a quadruped, for instance, though we may look at the eye of a molluscous animal or of an insect, as a proofhow simple an organ will serve some of the ends of vision; and at the eye of a fish as a nearer guide ofthe manner ofsimplifi-cation (Foundations, p. 130).

Another set of perhaps more legitimate difficulties arises in conjunction with what is now called *altruism - though the examples discussed by CD would not now be seen as cases of altruism: The nature or condition of certain structures has been thought by some naturalists to be of no use to the possessor, but to have been formed wholly for the good of other species; thus certain fruit and seeds have been thought to have been made nutritious for certain animals - numbers of insects, especially in their larval state, to exist for the same end -certain fish to be bright coloured to aid certain birds ofprey in catching them, etc. Now could this be proven (which I am far from admitting) the theory of natural selection would be quite overthrown; for it is evident that selection depending on the advantage over others of one individual with some slight deviation would never produce a structure or quality profitable only to another species [. . .]: the bright colours of a fish may be of some advantage to it, or more probably may result from exposure to certain conditions in favourable haunts for food, notwithstanding it becomes subject to be caught easily by certain birds. (Foundations, pp. 130-1).

The potential 'falsifiability' of natural selection, here stressed by CD, is a key reason why *Popper was wrong in considering this theory 'metaphysical' (as *creationism is). But then again, Popper is one of the many philosophers ofscience who derived most ifnot all their case studies from physics, and who therefore missed what Ghiselin (1969)called "the triumph of the Darwinian method".

Diodon spp. See Burrfishes.

Distribution CD used the distribution of animals and plants for much of his argument against special creation, and he was quite frustrated about the state of knowledge on the distribution of fishes, then insufficient for the type of inference he had in mind (see, for example, what we now know of *eel reproductive migrations).

This frustration shines through in the quotes below, taken from Notebook C (p. 243): many fish of Taiti found at <New> Isle de France:2 xx instance of wide range, where means of wide range3 work this out - L. *Jenyns, about my fish. NewZealand & New Holland fish very similar. -5 (nn. 2, 3, and 5 refer to Lesson (1826), pp. 27-8).

All the discussions <after> about affinity & and how one order first becomes developed & then another - (according as parent types are present) must follow after there is proof of the non creation of animals. - then argumen May be. - subterranean lakes, hot springs &c &c inhabited therefore mud wood be inhabited, then how is this effected by - for instance fish being excessively abundant (Notebook C, pp. 258-9).

Also: Mr Cuming informs me, that he has upwards of a hundred species of shells from the eastern Coast ofAfrica identical with those collected by himself at the Philippines and at the eastern coral-islands of the Pacific Ocean: now the distance from these islands to Eastern Africa is equal to that from pole to pole. Under similar circumstances Dr. Richardson has found that fishes have immense ranges. (Darwin 1846, p. 204 in Collected Papers I; Richardson 1846, pp. 190, 191; the naturalist Hughes Cuming [1791-1865] returned to England in 1839 after 20 years collecting living and fossil shells in the Pacific; Garber 1994).

Gradually, these large ranges, however, turned from a liability into an asset for his theory, and led CD to generate a bold prediction: Geologists finding in the most remote period with which we are acquainted, namely in the *Silurian period, that the shells and other marine *productions5S in North and South America, in Europe, Southern Africa, and Western Asia, are much more similar than they now are at these distant points, appear to have imagined that in these ancient times the laws of geographical distribution were quite different than what they now are: but we have only to suppose that great continents were extended east and west, and thus did not divide the inhabitants of the temperate and tropical seas, as the continents now do; and it would then become probable that the inhabitants of the seas would be much more similar than they now are. [. . .] Many fish, I may add, are also common to the Pacific and Indian Oceans. (Foundations, p. 179; n. 59 reads: D'Orbigny shows that this is not so.; Orbigny 1834-47, see also Geoffroy-Saint Hilaire and Blainville 1834).

This bold prediction nicely describes the Tethys Sea, which ranged in time from the lower Cambrian to the later Tertiary, and in space from what are now the Eastern Central Pacific and the Caribbean (the Isthmus of *Panama did not exist then) in the west, via the Indian Ocean, to Southeast Asia in the east. The Tethys Sea was identified and named by Suess (1885), over 40 years after CD had suggested its existence (see Wegener 1966; Ekman 1967, Chapter iv; see also Bony fishes).

Divine intervention(s) CD's views on these were straightforward: I have reflected a good deal on what you say on necessity of continued intervention of creative power. I cannot see this necessity; & its admission, I think would make theory of nat. select. valueless. Grant a simple archetypal creature, like the *Mud-fish or

Lepidosiren, with the five senses & some vestige of mind, & I believe Natural selection will account for production of every Vertebrate animal. (Correspondence to C. Lyell, Oct. 20,1859).

The astute reader will note that this barb, directed against *creationism, also questions the 'contingent' evolution so strongly advocated by S. J. Gould in Wonderful Life (1989) and many of his other writings (including Gould 2002), even if CD himself noted that! fish can never become a man (Notebook B, p. 227). The point is that fish, over the eons, did 'become' men and women.

Dogfish Common name used for small sharks, notably the widespread Squalus acanthias Linnaeus, 1758.

CD discussed their low fecundity, in contrast to that of *Cod: The Picked Dog-fish (Squalus acanthias) actually swarms on many coasts & yet is said to lay only six *eggs; whereas the Cod-fish sometimes lays above three million & a half.1 (BigSpecies Book, pp. 206-7; n. 1 refers to Yarrell (1836), Vol. 2, p. 401, for the high abundance, and to Fleming (1822), Vol. 2, p. 356, who cites Rondelet (1554); see also Eggs of fishes).

CD described as follows two specimens of 'dogfish' - probably S. acanthias - from Tierra del Fuego Colour pale, 'Lavender purple,' with cupreous gloss, sides silvery; above with irregular quadruple chain of circular and oblong snow white spots; tip of dorsal and *caudal blackish, under part of caudal reddish, iris pearly white. Length of old specimen tip to tip 2 feet 3 inches. breadth from tip of pectoral to tip ofother 8 inches. Young specimen out ofbelly; with it is posterior spine ofold specimen. (Fish inspirits, no. 840; also no. 882).

Dohrn, Anton German naturalist (1840-1909), whose correspondence with CD (Groeben 1982) shows that he failed to get across his idee fixe that vertebrates evolved from flipped-over annelid worms (Dohrn 1875) (Fig. 8), a notion

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