176 SCIENCE REMAKING THE WORLD 



in discovering experimentally how one species may 

 produce another one. The fact of evolution is estab- 

 lished, but the whole story of evolution must remain an 

 inference. 



PRESENT STATUS OF EVOLUTION. Only a very 

 general statement is possible, since a full statement 

 would involve an extensive discussion. The experi- 

 mental study of evolution has led to the development 

 of the field of genetics, a subject which has grown with 

 remarkable rapidity. It is genetics which must un- 

 cover the machinery of evolution, which of course is 

 fundamentally a matter of inheritance. The facts 

 thus far uncovered indicate complexities which were 

 not realized before, but which should have been anti- 

 cipated, for inheritance with its resulting evolution 

 represents the most complex biological situation imag- 

 inable. 



The present status of evolution as a body of doctrine 

 may be said to be in a state of flux, out of which the 

 truth will emerge eventually. Any meeting of biolo- 

 gists at which evolution is discussed will disclose con- 

 siderable diversity of opinion. It is evident, of course, 

 that whatever produces variation furnishes a basis for 

 evolution. But what produces variation? Environ- 

 ment is one factor, direct or indirect; sex is another 

 factor, especially when strains are crossed; and still 

 other factors might be cited. Any factor claimed to in- 

 duce variation must stand the test of genetics, with 

 its cytological background. Variations, however pro- 

 duced, are of two general kinds, as indicated by be- 

 havior,, namely, the so-called continuous variation of 



