430 DEVELOPMENT AND EMBRYOLOGY. 



that the similar bones in the hand of a man, wing of ?«. bat, 

 and fin of a 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 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 pro- 

 vide 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 man- 

 ner 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 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 

 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 must be consid- 

 ered as lower in the scale than the larva, as with certain 

 parasitic crustaceans. To refer once again to cirripedes : 

 the larvae in the first stage have three pairs of locomotive 

 organs, a simple single eye, and a probosciformed mouth, 

 with which they feed largely, for they increase much in. 

 8ize. In the second stage, answering to the chrysalis stag* 



