MILESTONES OF EVOLUTION 41 



The study of young mammals does, nevertheless, throw 

 a flood of light bearing on their ancestry, and illumines 

 no less the stages in the development of the particular 

 characteristics of this or that group of individuals. In 

 fact, the stages of development are so many signposts 

 along the pathway of evolution. 



As we have already pointed out, there is no common, 

 fixed, starting-point at which the study of young mammals, 

 or of any other young animals, begins : we cannot even 

 draw a hard-and-fast line between the embryonic and 

 the post-embryonic life, for some animals are born at 

 relatively much earlier stages of development than others, 

 as we have seen in the cases of the kangaroo and the 

 horse, for instance. 



But let us pass from the abstract to the concrete ; 

 selecting, by way of a commencement, some facts revealed 

 by a study of the teeth. Mammals differ from all other 

 tooth-bearing animals of to-day in that they never develop 

 more than two sets during their lifetime. The first is 

 known as the milk-dentition, because it is developed 

 towards the end of the suckling period, the second as 

 the permanent set. In this " milk-dentition " the teeth 

 are fewer in number and smaller in size than those of 

 the permanent set ; they may even be less complex in 

 structure than the permanent teeth. 



It has been supposed that these " milk "-teeth bear 

 no relation to more primitive, adult types of teeth, but 

 have been developed solely as temporary instruments for 

 dealing with solid food until the jaws have attained their 

 full size, and have consequently developed the necessary 

 space for the accommodation of the larger and more 

 numerous adult teeth. But this is only partly true. 

 These mUk-teeth do indeed represent the adult type of 



