56 GENERAL PRINCIPLES OF ZOOLOGY. 



for research. There is no other zoological theory which compares 

 with it ill value as a working hypothesis. To the many objections 

 which have been made that the theory is insufficiently 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 simjjle way and on a scientific basis. In this sentence 

 is given the merit of the theory, but at the same time also a limita- 

 tion of its ap)plicability. For on the one side the statement attrib- 

 utes 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. 

 On both sides no constant quantities are involved. Many investi- 

 gators see no necessity of reconciling paleontology and our knowl- 

 edge of plants and animals. To such, therefore, the Darwinian 

 theory proves just as little as any opposing theorj-. 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 jorobable, 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 first obtained credence through Darwinism, will be 

 Q, l)erinanenl landmark of zoological investigation. 



