50 EVOLUTION AND ANIMAL LIFE 



alike. This refers not only to individuals of different species 

 of plants and animals, but to individuals of the same species 

 and even (and this in a way is most important of all) to indi- 

 viduals born of the same parents. It is indeed this last condi- 

 tion that is the actual basis and fundamental beginning for 

 species change. That this variation does exist is absolute fact, 

 and there is no discussion of it. 



To what extent or degree, what parts of an organism are 

 chiefly affected, whether or no tliis variation shows a regularity 

 in its occurrence or a determinateness of tendency or direction, 

 whether or no this variation is based on inheritance and if so 

 in what degree of similaritvor iclentitv — all these and a dozen 

 other questions are the moot problems in connection with the 

 great factor variation. These are undecided things, which 

 means, on the whole, that variation, apart from the observed 

 and admitted actuality of the occurrence, is itself a great 

 evolution problem. 



The variation alone, however, presumably does not make 

 new species nor maintain lines of descent. If this variation is, 

 as it seems to be, almost unlimited in its range of appearance, 

 then as species are of definite character and number and as lines 

 of descent are even more definite and more limited as to number, 

 there must be some factor which determines what kinds or lines 

 of variation may or shall persist and what shall be extinguished. 

 Is there something incident to the causes of variation that 

 determines what lines of descent shall be established by it or 

 based on it, or is there some added factor which, having no 

 control over the initial appearance of variation, has absolute 

 control over its persistence and headway? Darwin^s factors of 

 selection, more particular^ natural selection, is the explanation 

 of this control offered in the famous "QrJgin of Species.^' And 

 natural selection has been in the minds of biologists until to-day, 

 at least, undoubtedly that factor in evolution which has been 

 believed to have the chief control in the forming of species and 

 the direction of descent lines. 



But in reference to this particular factor three schools of 

 biologists have gradually grown up; namely, first the school 

 headed by Weismann, who has believed and contended that 

 natural selection is almost the only factor which, on a basis of 

 fortuitous, that is, uncontrolled, variation, has produced the 

 species and lines of descent as we know them; second; the 



