MUTUAL AFFINITIES OF ORGANIC BEINGS 397 



no direct relation to their conditions of existence. We cannot, for 

 instance, suppose that in the embryos of the vertebrata the pecu- 

 liar, 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 

 beheve 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. 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 provide for itself. 

 The period of activity may come on earher 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 labor during the 

 different stages of development, as when the same larva has dur- 

 ing 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 than 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 pedun- 

 culated and sessile, though differing widely in external appear- 

 ance, have larvae in all their stages barely distinguishable. 



The embryo in the course of development generally rises in or- 

 ganization. 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 considered 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 loco- 



