32 MAMMALIAN DENTITION 



first pointing out the transformation of one form of animal into 

 another. Ten years after the publication of Darwin's Origin of 

 Species Waagen showed that minute and inconspicuous changes 

 of form occur constantly in the history of a race. These 

 changes which he called mutations are heritable and gradually 

 accumulate in successive generations until they become recog- 

 nizable. The most important of Waagen 's observations was 

 that these accumulations of mutations occur always in definite 

 directions. This means that there is a certain limited number 

 of types of response which it is possible for the germ-plasm to 

 exhibit. That these responses are adaptive has been clearly 

 shown by Osborn. One scarcely likes to use either of the 

 terms ordered response or choice of response in this connection 

 because of the anthropomorphic significance which these 

 phrases have come to possess. The former suggests a creative 

 plan and the latter a mentality on the part of the organism by 

 no means implied in these pages and carrying us further into 

 the realm of philosophy than we can go in this volume. We 

 cannot expect to understand why the germ-plasm should re- 

 spond in a particular manner until we attain more complete 

 knowledge than we have today of the interaction of the forces 

 of heredity with the various factors composing the environ- 

 mental stimulus. Throughout the succeeding pages, however, 

 the reader will find in the evolution of teeth ample evidence 

 of limited and definite types of response comparable with those 

 to which attention was first drawn in Invertebrates by Waagen. 



