280 MAMMALIAN DENTITION 



a variation and a mutation, definitions of both of which have 

 been given upon page 31. Each individual of every genera- 

 tion in all species shows innumerable mutations throughout 

 its body. Some of these are without significance upon the life 

 of the organism; some are distinctly detrimental and under 

 the operation of the Law of Natural Selection will result ulti- 

 mately in the extinction of the type; others again bring the 

 individual more closely into harmony with its environment 

 and this in turn results in the dominance of the type, perhaps 

 ultimately to the exclusion of other types, as for example in 

 the paleontological history of the Horse. But, it may be ob- 

 jected, the evolution of hypsodonty in the Hyrax has not re- 

 sulted so far in extinction of the brachyodont species al- 

 though hypsodont forms have been present ever since the 

 Miocene. This we cannot explain. Hypsodont and brachyo- 

 dont species of Hyrax exist together under what are appar- 

 ently the same conditions in the same locality. We must 

 admit that there are controlling the modification of tooth forms 

 factors of which Ave are utterly ignorant. 



We iioav find ourselves in the domain of philosophy arguing 

 against the mechanical interpretation of things. One of the 

 tenets of Bergson's doctrine of Creative Evolution is that there 

 is a constant progression in life, the evolution of new features. 

 Nowhere in the body is this doctrine more strikingly exempli- 

 fied than in the teeth which are among the most progressive of 

 bodily structures and nowhere is the inadequacy of a mere 

 mechanistic interpretation more obviously unsatisfactory. To 

 quote Darbishire reflecting Bergson's metaphor: "A rolling 

 stone gathers no moss. That is because a stone is a lifeless 

 thing. But a living thing whose life is made up of its experi- 

 ence is like a snowball Avhich is rolled in the snow; it increases, 

 as it is rolled, with the snow that it gathers, until it is big 

 enough to make a snow-man of. * * * When we look at 

 life we are in the presence of the continual creation of the 

 absolutely new." It is not to be expected that a single muta- 

 tion appearing in a single individual is ever going to be able to 



