440 EMBRYOLOGY. Chap. XIII. 



tions of existence. We cannot, for instance, suppose 

 that in the embryos of the vertebrata the peculiar 

 loop-like course of the arteries near the branchial slits 

 are related to similar conditions, — in the young mammal 

 which is nourished in the womb of its mother, in the 

 egg of the bird which is hatched in a nest, and in the 

 spawn of a frog under water. We have no more reason 

 to believe in such a relation, than we have to believe 

 that the same bones in the hand of a man, wing of a 

 bat, and fin of a porpoise, are related to similar condi- 

 tions of life. No one will suppose that the stripes on 

 the whelp of a lion, or the spots on the young blackbird, 

 are of any use to these animals, or are related to the 

 conditions to which they are exposed. 



The case, however, is different when an animal during 

 any part of its embryonic career is active, and has to 

 provide for itself. The period of activity may come on 

 earlier or later in life ; but whenever it comes on, the 

 adaptation of the larva to its conditions of life is just as 

 perfect and as beautiful as in the adult animal. From 

 such special adaptations, the similarity of the larvae or 

 active embryos of allied animals is sometimes much ob- 

 scured ; and cases could be given of the larvae of two 

 species, or of two groups of species, differing quite as 

 much, or even more, from each other than do their adult 

 parents. In most cases, however, the larvae, though 

 active, still obey more or less closely the law of com- 

 mon embryonic resemblance. Cirripedes afford a good 

 instance of this : even the illustrious Cuvier did not per- 

 ceive that a barnacle was, as it certainly is, a crustacean ; 

 but a glance at the larva shows this to be the case in an 

 unmistakeable manner. So again the two main divi- 

 sions of cirripedes, the pedunculated and sessile, which 

 differ widely in external appearance, have larvae in all 

 their several stages barely distinguishable. 



