EMBRYOLOGY 251 



Other, however different the adults may become; and so 

 it is with very many other animals. A trace of the law 

 of embryonic resemblance occasionally lasts till a rather 

 late age: thus birds of the same genus, and of allied 

 genera, often resemble each other in their immature plu- 

 mage; as we see in the spotted feathers in the young of 

 the thrush group. In the cat tribe, most of the species 

 when adult are striped or spotted in lines; aad stripes or 

 spots can be plainly distinguished in the whelp of the 

 lion and the puma. We occasionally though rarely see 

 something of the same kind in plants; thus the first 

 leaves of the ulex or furze, and the first leaves of 

 the phyllodineous acacias, 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, 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 wliich 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 sim- 

 ilar conditions of life. No one supposes that the stripes 

 on the whelp of a lion, or the spots on the young black- 

 bird, are of any use to these animals. 



The case, however, is different when an animal during 

 any part of its embrj'^onic career is active, and has to 



