374 THE POPULAR SCIENCE MONTHLY. 



alike in all cases. Before answering this question, we must call the 

 attention of the unscientific reader to a familiar fact which will throw 

 great light upon the matter. 



The animals with which we are most familiar, the mammals and 

 birds, are born in substantially the form which they will have when 

 they reach maturity. They breathe the same medium ; they employ 

 the same organs of locomotion, in the same way ; they require the 

 same or nearly the same kind of food, and their habits and surround- 

 ings are the same as they will be during mature life, or at least the 

 differences are slight and insignificant, and the adult is little more 

 than the young animal grown to its full size, and with sexual charac- 

 teristics, and able to reproduce its kind. But we must recollect that, 

 in the greater part of the animal kingdom, this is not the case. In by 

 far the greater part of the species of animals the rule is that the 

 newly born young is very different, in structure, in habits, surround- 

 ings, and needs, from the adult, and its passage to the adult form is 

 not simply a process of growth, but a process of great change in every 

 particular. 



The young butterfly crawls over the plant on which it is born, and 

 finds an abundant supply of proper food in the green leaves, which it 

 cuts to pieces with its strong, scissor-like jaws. Its capacious digestive 

 tract is fitted for dealing with great quantities of bulky but very 

 slightly nutritious food ; and its enemies, dangers, and means of de- 

 fense are very different from those of the adult winged insect, which 

 is furnished with highly developed sense-organs, and flies from place 

 to place in search of the highly concentrated liquid food adapted for 

 sucking up through the proboscis which has replaced the cutting jaws 

 of the young ; and we must recollect that the life-history of the butter- 

 fly, so far as its great changes are concerned, is a type of the life of 

 numbers of other animals, for nearly all the invertebrates pass through 

 a metamorphosis. 



Whenever young animals are left to shift for themselves, without 

 parental protection, they are compelled to struggle for existence with 

 a host of competitors and enemies ; and in all cases where the struct- 

 ure and habits of the young differ from those of the adult, a variation 

 in the young animal may be as important for the welfare of the spe- 

 cies as one in the adult, and may, therefore, be seized upon and per- 

 petuated by natural selection, and in this way the young stages of 

 two closely related species may be modified in different directions 

 until they become quite different from each other, while the adults 

 may remain essentially alike ; and as natural selection may act in such 

 a way as to modify the life-history of an organism at any stage of its 

 existence, there is no limit to the secondary changes which may thus 

 be brought about. A larva may acquire new organs, or it may lose 

 old ones ; the order in which organs are acquired may be modified ; 

 stages of development may be dropped out of the series, or new ones 



