242 DEVELOPMENT AXD EMBRYOLOGY. [Chap. XIV. 



are pinnate or divided like the ordinary leaves of the 

 legiiroinosae. 



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 

 conditions of existence. We cannot, for instance, 

 suppose 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 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 similar bones in the hand of a 

 man, wing of a bat, and fin of a porpoise, are related to 

 similar conditions of life. ISTo 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 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. 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 larvae of some insects belonging to 

 very different orders, and on the dissimilarity of the 

 larvae of other insects within the same order, according 

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

 similarity of the larvae of allied animals is sometimes 

 greatly obscured ; especially when there is a division of 

 labour during the different stages of development, as 



