DARWIA'. I'jy 



Naturae." Yet the field shows no signs of exhaus- 

 tion. As these volumes stand on the shelf together 

 side by side, it is easy to see that the later volumes 

 are the thickest ; and the '* Record " for the present 

 year is the largest of all. And what is true of 

 the increase of knowledge in Systematic Zoology, 

 is even more marked in the case of Botany. Such 

 then is the variety in the life of the globe, — a 

 variety of which Linnaeus and his successors had 

 never dared to dream. 



And yet great as this variety is, there are only 

 a few types of structure among animals and plants, 

 after all, — some eight or ten general modes of 

 development — and all the rest are minor varia- 

 tions from these few types. The law which tends 

 to keep the species uniform is the law (or the fact, 

 which we cannot wholly explain) that living beings 

 resemble their ancestors. And as each living 

 being has twice as many ancestors behind it as 

 either its father or its mother had, so does the 

 influence of remote ancestry diminish with each 

 succeeding generation. With this law or fact of 

 unity through heredity goes another law (or fact) 

 that no two living beings are ever exactly alike, 

 small variations of all sorts are constantly appear- 

 ing; and of the multitude of these variations some 

 few will be preserved, and favorable circumstances 

 will cause them to be repeated and augmented. 



Life then is changing on the earth, in spite of 

 the action of heredity. Past life must differ from 

 present life, and so in the life of past ages we may 

 trace the ancestry of the life of to-day. The agen- 

 cies which thus gradually modify species are 



12 



