Studies on variation and selection. jie 



sporting varieties'" belong to this group, though under the name 

 *'eversporting varieties" there are undoubtedly included the most 

 ■diverse cases, such as complicated cases of segregation. 



We have now seen, that contiauous variation may either be 

 ■caused by an underlying variation of the genotype, and in so far be 

 "inheritable", or by a variation in the non-genetic developmental 

 factors and in so far be non-inheritable. 



Also, discontinuous variation may have its cause in a variation 

 of the genotype, or in a vaiiation of the environment in the widest 

 sence, and correspondingly may be either inheritable or otherwise. 



III. Selection in genotypically homogeneous groups of 



organisms. 



By considering only the effect of selection upon the characters 

 of the individuals, and by looking upon this effect as of fundamental 

 importance, it was unavoidable that Zoologists and Botanists would 

 reach conclusions, which at first sight look irreconcilable. 



Some Botanists have declared, that selection of small differences 

 ■can have no effect, or at least no permanent effect, and they have 

 generalized this statement, forgetting to add "in self-fertilized plants". 



Zoologists on the other hand have declared that selection of small 

 differences must have been an important factor in species-formation, 

 because they found it to be so in animal-breeding. They too have 

 generalized, and have forgotten to add "in allogamous organisms". 



In a general way we can say, that selection of small differences 

 is an important factor in breeding and in species-formation, in those 

 oases and in so far as these small differences are caused by corre- 

 sponding differences in the genotype, by presence and absence of 

 genes, for which the population is not pure, and not, when these 

 differences are caused by non-genetic developmental factors. Selection 

 therefore has very speedy and permanent effect in a population 

 consisting of a mixture of two or more pure types which have no 

 intercourse sexually, such as pure clones, or pure types of self-fertilized 

 plants homozygous for all their genes, or again in a population con- 

 sisting of two or more non-mixing types of animals, e. g. asexually- 

 propagating types or species which do not mate together, such a 

 Mus rattits and Mas norvegicus, or horses and cattle. 



Selection has far less speedy, and less permanent effect, in popu- 

 lations, consisting of organisms which can interbreed and which there- 



