94 UNIVERSAL EVOLUTION 



appear at one time as in mutation? Reversions may 

 also prove that variations, not useful, may be carried, 

 like rudimentary organs, for several generations, and 

 then suddenly disappear. 



Natural selection acts as a sieve; it does not sift 

 out the best variations, but it simply destroys the 

 larger number of those, which are. from some cause or 

 another, unfit for their present environment. In this 

 way, it keeps the strain up to the required standard, 

 and, in special circumstances, may even improve them. 

 (De Vries.) 



SPONTANEOUS GENERATION. There are many authors 

 who write on biology and evolution in a very learned 

 way, who discredit natural selection, as a method of 

 evolution. These dwell very learnedly on the mode of 

 appearance of living units from non-living matter, 

 and the innate tendency of living matter to variation. 

 They believe that archebiosis and heterogenesis fre- 

 quently occur. "Variation" is only another term to ex- 

 press these two modes of bio-genesis. Archebiosis is 

 really spontaneous generation, or the formation of 

 protoplasm from inorganic elements. Heterogenesis is 

 the production of a different, and more complex 

 form, and function, from that of its progenitors. 

 Both of them, therefore, can be termed methods 

 of variation. "Organic polarity" and "mutation" 

 belong to the same category. They are theoretical 

 causes of variation, and simply express an unknown 

 process of change, by, either, continuous, or discon- 

 tinuous growth. It is so with "seasonal dimorphism." 

 There are three other factors of evolution mentioned 

 by Bastian, and also by Darwin, viz., "sexual selec- 

 tion," the "effects of use and non-use," and "the direct 

 influence of external conditions." These are all 



