^8 DEVELOPMENT AND EMBRYOLOGY. 



bryos of the vertebrata the peculiar loop-like courses of the 

 arteries near the branchial slits are related to similar con- 

 ditions — in the young mammal which is nourished in the 

 womb of its mother, in the Qgg 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, vring of a bat, and fin of porpoise, are related to 

 similar 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 embryonic career, is active, and has to pro- 

 vide for itself. The period of activity may couia on 

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

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

 fect and as beautiful as in the adult animal. In how im- 

 portant 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; es23ecially when there is a division of 

 labor during the different stages of development, as 

 when the same larva has during one stage to search for 

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

 attachment. Cases can even be given of the larvae of allied 

 species, or groups of species, differing more from each 

 other than do the adults. In most cases, however, the 

 larvae, though active, still obey, more or less closely, the 

 law of common embryonic resemblance. Cirripedes afford 

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

 perceive that a barnacle was a crustacean: but a glance at 

 the larva shows this in an unmistakable manner. So again 

 the two main divisions of cirripedes, the pedunculated and 

 sessile, though differing widely in external appearance, 

 have larvae in all their stages barely distinguishable. 



The embryo in the course of development generally rises 

 in organization. I use this expression, though I am aware 

 that it is hardly possible to define clearly what is meant by 



