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 
