242 DEVELOPMENT AND EMBRYOLOGY. [Chap. XIV. 



are pinnate or divided like the ordinary leaves of the 

 leguminosse. 



The points of structure, in which the embryos of 

 widely different animals within the same class resemble 

 each other, often have no direct relation to their con- 

 ditions of existence. We cannot, for instance, sup- 

 pose that in the embryos of the vertebrata the peculiar 

 loop-like courses of the arteries near the branchial slits 

 are related to similar conditions, — in the young mam- 

 mal 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 be- 

 lieve that the similar bones in the hand of a man, wing 

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

 ditions of life. No one supposes that the stripes on the 

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

 are of any use to these animals. 



The case, however, is different when an animal dur- 

 ing 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. In how 

 important a manner this has acted, has recently been 

 well shown by Sir J. Lubbock in his remarks on the 

 close similarity of the larvje of some insects belonging to 

 very different orders, and on the dissimilarity of the 

 larvEe of other insects within the same order, according 

 to their habits of life. Owing to such adaptations, the 

 similarity of the larvse of allied animals is sometimes 

 greatly obscured; especially when there is a division of 

 labour duting the different stages of development, as 



