FACTORS AND MECHANISM OF EVOLUTION 49 



general theory of organic evolution and the particular doctrine 

 of the descent of man from the lower animals. For it was 

 Darwin who really proved these things to be realities. But in 

 biology to-day Darwinism is the name which refers to certain 

 particular causal factors or determining agents in the actual 

 production and control of the transmutation of species and the 

 progress and direction of the lines of descent. And the modern 

 scientific adverse criticism of Darwinism which is beginning to 

 find its echoes in popular literature must never be mistaken to be 

 disparagement or adverse criticism of the doctrine of descent, 

 the law of organic evolution. So we are not in this book dis- 

 cussing the probabilities of the truth or untruth of evolution 

 nor presenting evidences or argument to justify a belief in the 

 doctrine. As the days have long passed when the shape of the 

 earth, or the behavior of the members of the solar system, 

 was a fit subject for debate, so the days are now by when the 

 truth or falsity of the law of organic descent is a debatable 

 thesis. The earth is subspherical, the planets revolve about 

 the sun, and species of organisms descend from other species. 



But in what particular way, or as the effect of what particular 

 causal factors, this descent or transformation of species, that is, 

 kinds of organisms, comes about, here there is unlimited field 

 for debate and polemic, for hypothesis and investigation, for 

 deduction and determination. It is the factors of organic 

 evolution, the factors of each of the particular phases or aspects 

 of evolution phenomena, that are the subject of present-day 

 biological study and discussion. The mechanism and method 

 of evolution is' a subject, with its score of moot questions, its 

 enormous opportunity and inspiration, for fact gathering, fact 

 arranging, arid fact interpreting. So in biological science to- 

 day, no less but even more than in those first exciting days 

 after the "Origin of Species/' the subject and problems of 

 evolution are the inspiriting and absorbing matters which chiefly 

 occupy the attention of biologists. What these factors are that 

 compose the chief subject of present-day evolution study and 

 discussion may be summarily set out in the present chapter. 



In the first place it is obvious that there can be no transfor- 

 mation or change of species unless there is an ever-present actual 

 variation. By variation is simply meant, in the larger sense, 

 that no two individual organisms in the world, nor for that 

 matter any two that have ever been in the world, are exactly 



