68 THE NEXT GENERATION 



may stop using old ones. If it stops using any part of its 

 body, that part may be inherited as a rudiment. 



3. The embryo of many vertebrates tells by its different 

 parts what the history of the evolution of its ancestors has , 

 been. 



For example, when we find rudimentary teeth in the front 

 upper jaw of an embryo calf, we know that once upon a time 

 the ancestors of this calf had well-developed upper front teeth 

 and used them for chewing. When we find rudimentary 

 legs hidden under the flesh of full-grown whales, we know 

 that, in the ages of the past, whale ancestors used legs instead 

 of fins for locomotion. 



So, too, with gill-slits. When we find rudiments of these in 

 any embryo, we know that somewhere back in bygone ages 

 the ancestors of this particular embryo lived in water and 

 breathed through gill-slits. 



Evidently, then, each rudiment is nothing less than a 

 signboard a reminder of the road by which developing 

 creatures have traveled from the past to the present. Each 

 is a so-called ancestral reminiscence. Each declares that 

 in the struggle for existence this organ or that had to be 

 given up, and that other organs were developed instead. 



Our modern whale tells the story of a double change. To- 

 day he is a queer combination a mammal that lives in 

 water and uses lungs for breathing. But his rudimentary 

 legs and his rudimentary teeth prove that at some time in 

 the past his ancestors were out-and-out land animals that 

 roamed the fields on sturdy legs and used good-sized teeth 

 for chewing. 



Also, by his embryonic gill-slits the same whale tells us 

 that even before he was a land animal his ancestors were 

 water animals with gill-slits for their breathing apparatus. 



