68 Evolution and Adaptation 



or ridges, develop in the mouth of the embryo ; two in the 

 upper, two in the lower, jaw. These ridges appear to corre- 

 spond to those of reptiles and mammals, from which the teeth 

 develop. It may be said, therefore, that the rudiments of 

 teeth appear in the embryo of the bird. This might be inter- 

 preted to mean that the embryo repeats the ancestral reptilian 

 stage, or, perhaps, the ancestral avian stage that had teeth in 

 the beak ; but since only the beginnings of teeth appear, and 

 not the fully formed structures, this interpretation would 

 clearly overshoot the mark. 



The embryo of the baleen whale has teeth that do not 

 break through the gums and are later absorbed. Since the 

 ancestors of this whale probably had teeth, as have other 

 whales at the present time, the appearance of teeth in the 

 embryo has been interpreted as a repetition of the original 

 condition. Some of the ant-eaters are also toothless, but 

 teeth appear in the embryo and are lost later. In the rumi- 

 nants that lack teeth in the front part of the upper jaw, e.g. 

 the cow and the sheep, teeth develop in the embryo which 

 are subsequently lost. 



One interpretation of these facts is that the ancestral 

 adult condition is repeated by the embryo, but as I have 

 pointed out above in the cases of the teeth in whales, since 

 the teeth do not reach the adult form, and do not even break 

 through the gums in some forms, it is obviously stretching 

 a point to claim that an adult condition is repeated. More- 

 over, in the case of the birds only the dental ridges appear, 

 and it is manifestly absurd to claim in this case that the 

 ancestral adult condition of the reptiles is repeated. 



That a supposed ancestral stage may be entirely lost in 

 the embryo of higher forms is beautifully shown in the devel- 

 opment of some of the snakes. The snakes are probably 

 derived from lizardlike ancestors, which had four legs, yet 

 in the development the rudiments of legs do not appear, and 

 this is the more surprising since a few snakes have small 



