specific Resemblances at all Stages of Life 125 



frog's Qgg and a fourth will develop in exactly the same way 

 as our first, and also the fifth and the tenth and the thou- 

 sandth. While a frog's tg^ does not look like a frog, or even 

 like a tadpole, it is nevertheless all frog and only frog. It 

 is as much a part of the " species " as is the adult. 



Specific Resemblances at all Stages of Life 



It may seem childish to repeat this. Of course, you will 

 say, one frog develops as does the next one. One chicken 

 develops like another one. One human being develops like 

 another human being. We asume this as a matter of course. 

 But why? We do not pretend to know just what makes the 

 animal change day by day as it does, but we are quite sure 

 that the sameness of the members of a species extends to their 

 development as well as to their adult form and structure. 

 A frog is like other frogs at every stage of its being. A clam 

 is a clam at every stage of its being. 



Just so. And although nobody has seen all the frogs or 

 all the clams or all the hens at every stage, it is common sense 

 to think in these terms. If we have seen the caterpillar of 

 the tomato worm we assume that the next similar tomato 

 worm is also the larva of the same kind of insect. If we 

 have seen the grub of the June beetle we assume that the 

 next similar grub is also the larva of the June beetle. In 

 the same way we recognize hens' eggs as hens' eggs even if 

 we were not present when they were laid, even if we do not 

 wait to see into what they will hatch. 



These facts are so familiar that we are apt to overlook 

 their implications. Yet they have an important bearing on 

 the question of relationships among different kinds of ani- 

 mals or plants. To the extent that we accept descent from 

 a common ancestor for all the June beetles or for all hens or 

 for all green frogs, we accept of course the same relationship 

 for the June bug larva or for green frog tadpoles. If we 

 knew the tadpoles only and never the adult frog, we should 

 never hesitate to think of them as the " same kind " of ani- 

 mals, and we should infer from their resemblances a common 



