HISTORY OF ZOOLOGY 49 



it can only be replied that it is the only theory which agrees with our ex- 

 periences and explains these in a simple way and on a scientific basis. 

 In this sentence is given the merit of the theory, and also a limitation of its 

 applicability. For on the one side the statement attributes the merit in the 

 applicability of the system to the necessity of the human mind for simple 

 explanations of the facts of natural science, and on the other hand it makes 

 the degree of correctness dependent upon the state, whatever it may be, 

 of our knowledge. Many investigators see no necessity of reconciling 

 paleontology and our knowledge of plants and animals. To such the 

 Darwinian theory proves just as little as any opposing theory. Meanwhile 

 thoughtful naturalists will keep in mind that our knowledge of nature is 

 making considerable advances, and is visibly becoming wider and deeper. 

 It is possible, even probable, that these advances will lead to many modi- 

 fications of the theory. The conception of the way in which forms have 

 developed from one another admits, as the mutation theory shows, 

 of very different expressions. On the other hand, we can affirm with 

 great certainty that the principle of descent, which first obtained cre- 

 dence through Darwinism, will be a permanent landmark of zoological 

 investigation. 



