134 Plant Genetics 



If self-sterility is useful to the plant the nature of its 

 usefulness should be considered. The situation may 

 be outhned as follows. The first plants used only 

 asexual reproduction, and this has proved to be a very 

 simple and effective method. Why then was sex ever 

 developed? It will be remembered that Weismann 

 was the first to answer this question clearly, claiming 

 that sex was developed to secure greater variation. 

 Greater variation is desirable of course to secure adapta- 

 tions to diverse conditions of living. It is well known 

 that asexual reproduction is the surest way to duplicate 

 the parent form and would result in uniform plants 

 generation after generation. If a species remains uni- 

 form within certain narrow hmits it will be impossible 

 to meet changing conditions. This will result in two 

 evils: (i) if conditions are changed where the uniform 

 species is growing the species will be exterminated; 

 (2) even if the local conditions remain the same the 

 species will be unable to spread into new localities where 

 the conditions are different. If, on the other hand, a 

 species shows wider variations, certain of these might 

 fit the new conditions. Such variations will survive 

 when conditions change and would always solve the 

 problem of migrating into new habitats. For these 

 reasons variation is necessary to the general success of 

 the species. 



This increase in the range of variations, therefore, is 

 the significance of the introduction of sex, for the fusion 

 of two germ plasms in the sex act greatly increases 

 chances for variation. Weismann thought that sex 

 secured the small individual variations which are so 

 common in any species, but now most such variations 



