396 DEVELOPMENT Chap. XIII. 



We cannot, for instance, suppose tliat in the embryos of the 

 vcrtebrata the pccuhar loop-like courses of the arteries.near the 

 branchial slits are related to similar conditions — in the young 

 mammal wliich is nourished in the womb of its mother, in the 

 c<rg 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 fm of a por- 

 j)oise, are related to similar conditions of life. No one sup- 

 poses tliat the stripes on the whelp of a lion, or the spots on 

 the young blackbird, are of use to these animals. 



Tlie 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 larvne of some insects belonging 

 to very different orders, and on the dissimilarit}^ of the larva; 

 of other insects within the same order, according to their 

 habits of life. From such adnptations, especially when they 

 im]>ly a division of labor during the different stages of develop- 

 ment, as Avheu the same larva has during one stage to search 

 for fooil, and has during another stage to search for a place of 

 attachment, the similarity of the lan'te of allied animals is 

 sometimes greatly obscured ; and cases could be given of the 

 larv;V3 of two species, or of two groups of species, differing 

 more from each other than do the adults. In most cases, how- 

 ever, the larv:c, though active, still obey, more or less closel}', 

 the law of common embrj^onic resemblance. Cirripedes afford 

 a good instance of this : even the illustrious Cuvier did not 

 perceive thai a barnacle was a crustacean ; but a glance at 

 the larva shows this to be true in an unmistakable manner. 

 So, again, the two main divisions of cirripedes, the peduncu- 

 lated and sessile, though differing M-idely in external appear- 

 ance, have larvoi in all their stages barely distinguishable. 



The cml>ryo in the course of development gencrall}- rises 

 in organization : I use this expressiDU, though I am aware that 

 it is hardly possible to deiinc clearly what is meant by the 

 organization being higher or lower. But no one probably will 

 dispute that the butterfly is higher than the caterpillar. In 

 some cases, however, the mature animal nmst be considered as 



