XXI.] EMBRYOLOGY. 267 



adaptive and the difference fundamental ; and there is no Anaio- 

 resemblance at first. Such are the bat's wing and the f^^^ °^^^ 

 insect's wing. And similarly, though the wing of the bat adaptive. 

 is precisely analogous with that of the bird, its homologies 

 are much nearer to those of the hand of man. 



To return to the subject of the flounders. We have Flounders, 

 seen that their peculiar unsymmetrical form is not an 

 embryonic character. But it is plainly an adaptive one; 

 it is to be referred to their peculiar mode of life, requiring 

 them to be able to swim as close to the bottom as possible ; 

 for which purpose they have acquired the habit of swim- 

 ming on one side instead of vertically, and have got their 

 forms modified to suit that habit. 



Of course, the distinction between fundamental and 

 adaptive characters is not an absolute distinction, but 

 admits of gradations. It is, however, generally true that 

 the fundamental characters are the embryonic ones ; and 

 that those characters which appear first in the course of 

 development are the least variable as between individuals, 

 species, genera, and classes. And if it is true, as is implied 

 in the development theory, that the most fundamental 

 characters are those which have been inherited from the Constancy- 

 remotest ancestry, and through the greatest number of °^ ^^'^'^^^" 

 generations, it follows that the comparative invariability characters, 

 of fundamental characters is a case of that law of habit, ^jfg'^i^^ ^f 

 in virtue of which the habits of the longest standing are habit, 

 the most tenacious and the least variahle. 



We have seen that nearly allied organisms undergo a 

 similar development; and that the characteristics of the 

 genus appear before those of the species, and the charac- 

 teristics of the wider group, generally, before those of the 

 narrower group. These laws are subject to exceptions, Excep- 

 some of which are very difficult to explain ; as when, in *'°'^®- 

 some cases, characters that belong to the species appear 

 earlier than others that belong to the genus.^ But there 

 is a large class of exceptions of a different kind, which, 



1 Stated on the authority of Agassiz, in Spencer's Principles of Biology, 

 vol. ii. p. 378. 



