HISTORY OF ZOOLOGY. 67 



The exposition of the descent theory has been elabo- 

 orated here in a rather detailed way, because in the history 

 of Zoology it is undoubtedly the most important feature. 

 No other theory has gained such a hold in the course of 

 the development of zoological investigation, none has pro- 

 pounded so many new problems and opened so many new 

 fields for research. There is no zoological theory which com- 

 pares with it in value as a working hypothesis. To the many 

 objections which have been made that the theory is insuffi- 

 ciently grounded, it can only be replied that in the present 

 state of our knowledge it is the only theory which agrees 

 with our experiences and explains these in a simple way 

 and on a scientific basis. In this sentence is given the 

 merit of the descent theory, but at the same time 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 ex- 

 planations 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. On both 

 sides no constant quantities are involved. Many investi- 

 gators see no necessity of reconciling paleontology and our 

 knowledge of plants and animals. To such, therefore, the 

 Darwinian theory proves just as little as any opposing the- 

 ory. Meanwhile reflecting naturalists will be obliged ever 

 to keep in mind that our knowledge of nature is making 

 considerable advances, and is visibly undergoing a widen- 

 ing and deepening. It is possible, nay, indeed probable, 

 that these advances will lead to many modifications of the 

 theory. For instance, the theory of the causes which 

 condition the formation of new species will undergo 

 numerous changes. On the other hand, we can affirm 

 with great certainty that the principle of descent, which 

 through Darwinism for the first time obtained credence, 

 will be a permanent landmark of zoological investigation. 



