Amphioxus See Lancelet

Anadromy(-ous) Refers to fish whose adults leave the sea and ascend rivers to spawn; most *salmon and *shads are anadromous. (See also Diadromy.)

Analogous organs Organs whose similarity depends upon similarity of function, as in the wings of insects and *birds. Such structures are said to be analogous and to be analogues of each other.(Origin VT,p. 430). Analogous organs constrast with 'homologous' organs, derived from the same body parts, such as the wings of *bats and birds, both ultimately derived from the forelegs of ancestral lower vertebrates. *Owen contributed greatly to the differentiation of these two concepts, a necessary step in the understanding of organic *evolution. CD discussed the implications ofanalogous organs under the heading Similar & peculiar organs in beings far remote in the scale of nature, viz.: -I have already alluded to the remarkable case of *Electric organs occurring in genera of fish, as in the *Torpedo & *Gymnotus almost as remote as possible from each other: but the organs differ not only in position, & in the plates being horizontal in one & vertical in the other, but in the far more important circumstance of their nerves proceeding from widely different sources1 (*BigSpecies Book, pp. 374-5; n. 1 cites Owen (1846) who, on pp. 217-18, describes two types of electric organ. However, Owen does not explicitly mention their nerves having different sources, though it may be implied.)

Also: According to our theory when we see similar organs in allied beings we attribute the similarity to common descent. But it is impossible to extend this doctrine to such cases, as those just given of the [...] Torpedo & Gymno-tus, the Echnida & Hedgehog &c, - excepting in so far that community of descent, however remote the common ancestor may have been, would give something in common to the general organisation. Just in the same way [...] we have seen that the occurrence of similar monsters in the most diverse members of the same great class may be attributed to a like organisation from common descent, being acted on by like *abnormal causes of change [...].

It is not, I think, at all surprising that natural selection should have gradually given a fish & a *whale something of the same forms, from fitting them to move through the same element; just as man in a small degree has given by his selection something in common to the form of the grey-hound & race-horse. A similar doctrine, I infer, must be extended to the above given remarkable cases of similar, though very peculiar & complex structures, in beings remote in the *scale of nature. Such cases are not common; & in some ofthem the parallelism, as we have seen in the electric organs offishes & in the eye of Cephalopod & mammal is not absolutely strict. (BigSpecies Book, pp. 375-6.)

Androgynous See Hermaphrodite.

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