THE MECHANICAL SYSTEM AND ITS LIMITS 359 



SO enables us to understand organisms on a mechanical 

 basis. 



We have already observed several times that there is 

 no longer any serious objection urged against our first 

 thesis, or against the general theory of evolution. It is 

 true that objections continue to be raised by a few men 

 of science. A book appeared recently in Germany that 

 spoke of " the break-up of the theory of evolution." But 

 the gist of the work was merely that we cannot establish 

 with certainty the descent of the various classes of 

 animals. The objection overlooks the fact that the 

 theory of selection has done enough as a scientific 

 theory when it shows in a general way the reasons 

 why we must suppose one species to have been evolved 

 from another; of this we have plenty of instances. 

 The fact that we cannot for the moment determine the 

 stages of development of the various classes makes no 

 difference to the general theory ; in fact, even if it were 

 true that we could not reconstruct, even in broad out- 

 line, the genealogical tree of an animal group, this 

 would not in the least affect the truth of the theory of 

 selection. The detailed efforts to construct the genea- 

 logical trees of the animals lie outside its province ; they 

 are not scientific at all in the same sense. They come 

 within the range of a totally different science — history. 

 We shall see this better if we consider the historical 

 side of the theory of evolution. 



Objections of this kind, therefore, do not injure the 

 theory of selection or of evolution ; they merely show 

 that in many cases it is difficult, if not impossible, to 

 trace the ancestry of an animal. But no reasonable 



