134 How Living Things Come to be What They Are 



The earliest mammals found in the rocks appear to have, 

 typically, forty-four teeth. Many of the modern mammals 

 have greatly reduced numbers. Among the rodents or 

 gnawers (rats, rabbits, squirrels, gophers, etc.) there is a gap 

 between the front gnawing teeth and the grinding teeth. 

 Yet the embryo of the squirrel shows beginnings of teeth in 

 this gap. In toothless whales, a study of the early stages 

 shows the presence of teeth in the jaws; but these are absorbed 

 before birth. 



Facts like these are taken to mean that whalebone whales 

 are related to the toothed whales; that rodents had ancestors 

 in which there was not a gap between the incisors and the 

 pre-molars. In the same way, we observe that the embyros 

 in many beetles bear appendages on all the segments or rings 

 of the abdomen, whereas the adult beetles bear legs only on 

 the thorax, like other insects. And w^e take these facts to 

 point to a relationship with a more primitive or generalized 

 type in which the parts of the body were not so sharply 

 differentiated. 



Embryos as Missing Links 



Increasing familiarity with the facts of animal structure 

 and development leads us to accept relationships within 

 groups that are obviously similar in form and structure, or 

 even in general plan of organization. We still have diffi- 

 culty, however, in considering animals of distinct types as 

 related. There is nothing in the structure of any backboned 

 animals to link up with the invertebrates. The segmented 

 worms may conceivably be related to the arthropods, since 

 the larval stages of most insects are very ** worm "-like. But 

 the other main branches appear to be so distinct that even the 

 biologist is puzzled to find possible transitions or connecting 

 links. The study of embryology has yielded some very 

 striking evidences of relationship. 



The mollusks, which include clams and oysters, snails 

 and periwinkles, squids and octopuses, represent a rather 



