THE FACTORS OF EVOLUTION 141 



give a satisfactory uniform explanation of the manner and method 

 by which the changes and development in the organic world 

 have taken place. But the doctrine of Darwin in its main points 

 still holds the field unrefuted, and honest criticism has found 

 itself restricted to pointing out the many weak parts of the 

 hypothesis of natural selection and to indicate that Darwin, and 

 in particular his successors, greatly over-estimated the importance 

 of natural selection, and that there are other causes which have 

 played a significant part in the development of the organic 

 world. But whatever may be the verdict of coming genera- 

 tions concerning the doctrine of this great man, it cannot detract 

 from his merits ; no one can rob him of the fame that it was 

 he who by his great and comprehensive work carried the Theory 

 of Descent to universal recognition and final victory. 



What are animal species? Are they really immutable, final, 

 natural units, as Cuvier states, or are they more or less arbitrary 

 definitions made by man in order to facilitate the study of 

 nature. Just as easy as it is strictly to define any species so 

 long as only a few specimens of it are known, so difficult and 

 yet even impossible becomes the task of an accurate definition 

 when we have to deal with numerous individuals, especially if 

 they belong to different geographical districts. It will always 

 happen that some will fit easily into the system whilst others 

 will diverge on many points. 



We see everywhere in Nature that the descendants, the 

 children, are more or less like their parents, and exhibit 

 generally the same specific features as the latter. On the other 

 hand, there is no doubt that no child is entirely and in all details 

 like its parents ; it will always differ from either to a greater or 

 lesser degree. In addition to the character of the species, it 

 bears its own individual character. This seems natural enough 

 when we remember that every higher organism owes its 

 existence to two different individuals and inherits charac- 

 teristics both from its father and mother. 



These individual fluctuations and deviations are called varia- 

 tions. Speaking generally, they move within very narrow limits, 

 revolving, as it were, round a fixed centre, the type of the 

 species. But under certain circumstances, when by accident or 

 owing to external unknown influences such variations accumulate 



