Nos. 618-619] THE BOLE OF BEPBODUCTIO? 



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strict mutationist would hardly maintain that evolution in 

 general has come about through tremendous changes in- 

 volving sterility between the mutant and the parent types. 

 It seems unnecessary to deny such possibilities; but the 

 weight of evidence is in favor of the majority of varia- 

 tions being comparatively small, changes in detail, tin- 

 very kind which are known to be Mendelian in their in- 

 heritance. 



Yet sexual reproduction in itself does not assure these 

 advantages, though they are based upon it. There must 

 be means for the mixture of germplasms. This oppor- 

 tunity was furnished originally by bisexuality. Then 

 came hermaphroditism, manifestly an economic gain, yet 

 on the whole unsuccessful except as functional bisexuality 

 was restored by self-sterility, protandry, protogyny or 

 mechanical devices which promoted cross-fertilization. 



The prime reason for the success of sexual reproduc- 

 tion then, as Weismann maintained, is the opportunity it 

 gives for mingling germplasms of different constitution 

 and thereby furnishing many times the raw material to 

 selective agencies that could possibly be produced through 

 asexual reproduction. Further, there are three minor ad- 

 vantages which rest upon the same mechanism. They are 

 minor advantages only when compared to the major, and 

 should not be passed by. 



Let us first consider heterosis, the vigor which accom- 

 panies hybridization. This phenomenon has long been 

 known. It is characteristic of first generation hybrids 

 both in the animal and vegetable kingdoms. It affects the 

 characters of organisms in much the same manner as do 

 the best environmental conditions. In other words, the 

 majority of characters seem to reach the highest de- 

 velopment in the first hybrid generation. The hybrid in- 

 dividual therefore holds some considerable superiority 

 over the individuals of the pure races which entered into 

 it. and is thereby the better enabled to survive and to 

 produce the multiplicity of forms which its heterozygous 

 factors make possible. The frequence of this phe- 

 nomenon, for it is almost universal, together with the fact 



