126 How Living Things Come to be What They Are 



ancestor. The student of embryology applies this same method 

 of reasoning in dealing with plant and animal embryos at 

 every stage of development. In so far as two embryos resem- 

 ble each other, they are taken to indicate a common ancestor » 

 We all know that a calf will grow into a cow, barring 

 accidents; that a colt normally becomes a horse. We have 

 seen puppy become dog in enough instances to feel war- 

 ranted in extending the general idea to other puppies. And 

 we have seen the transition from infancy or larva to adult 

 in enough instances to extend the general idea to all animals 

 — even to animals we have never seen. In passing from the 

 egg to the adult, each individual goes through a series of 

 stages in common with other members of the species. Again 

 we infer relationship proportional to the degree of resem- 

 blances at the various stages. An experienced breeder will 

 distinguish a Plymouth Rock from a Rhode Island Red while 

 the two chicks still look alike to the layman. Yet the layman 

 will have no difficulty in distinguishing the two chicks from 

 young ducklings. A dog fancier will distinguish various 

 breeds of dog before the puppies have opened their eyes al- 

 though all puppies may look alike to the rest of us. 



Relatives Have Similar Babies 



If we compare the successive stages in the development 

 of a frog with the early stages in the development of a toad, 

 we find correspondence in great detail. In most cases we 

 should not be able to distinguish the young tadpoles of dif- 

 ferent species, except perhaps by the size. They are in form 

 and proportion practically identical. To the extent then that 

 these animals are alike in their development we may consider 

 them related — even if we do not wait for the maturing of 

 the individual. For most people, in fact, frogs and toads 

 are indistinguishable terms. It is sufficient for most of us 

 to think that the toad is " a kind of frog." And it is suffi- 

 cient for most of us on seeing a tadpole to assume that it will 

 some day be " a kind of frog." But similar stages in develop- 



