Nature of Heredity and Problem of Mechanism 317 



between descendants and ancestors. Full consideration of 

 the question may be deferred for the moment, though atten- 

 tion must be called to the fact that resemblance even of 

 tliis sort, never, so far as we know, amounts to identity, even 

 though in many instances it is wonderfully close. Difference 

 is a no less universal rule than is similarity and from this 

 it results that science is absolutely j)rohibited from attempt- 

 ing to minimize the importance of either truth. The prob- 

 lems of organic likeness and difference are inseparable, and 

 those biologists are so far right who contend that heredity 

 has to do with both. In the interest of clear thinking how- 

 ever, it is necessary to recognize that resemblance is one 

 fact and difference another ; and that the idea of heredity 

 has rightly grown up in connection with the former. Hered- 

 ity and variation are not simply one fact and one problem. 

 They are two distinct, though essentially interrelated and 

 wholly inseparable facts and problems. This way of put- 

 ting the matter seems unescapable when we consider a cir- 

 sumscribed group of organisms, as for example the horses, 

 in which much is known about the ancestry in geological time 

 and the species and varieties now existing. The student of 

 such groups is alert for both points of resemblance and 

 points of difference between the members of the group. He 

 knows that his scientific integrity depends on his preserving 

 an exact balance of effort towards the two kinds of charac- 

 ters, and the degree of resemblance is his sole criterion of 

 degree of kindred. Particular note should be taken of the 

 difference of starting point relative to problems of heredity 

 held by students of genetics and by students of natural 

 organic groups. For the former the descent aspect of the 

 definition we have adopted is observationally known and is 

 the part in which their main interest lies, while for the 

 latter the genetic connection is, in a vast majority of cases, 

 forever beyond the reach of observation. With him it has 

 to be inferred or ignored. But what is his basis for in- 



