SELECTION. 191 



the structure of the flowers which ensures auto-fecundation. 



It is right to call these groups species. But we must recog- 

 nize that, whereas the purity of these species is very striking, 

 it does not determine their status as species. While new lines 

 of such plants are differentiating after a cross, they have not 

 yet attained to purity, but they are akeady species. Should 

 we restrict the term species to pure lines, or to some other 

 special category of species, it would become necessary to find 

 another terminology for all other species. 



It is evident, that for as long as it was thought that species 

 and all other groups of organisms had a natural tendency to 

 vary, the importance of selection for species-formation was 

 thought to be much greater, than we now know it to be. For 

 we know now, tht there is no tendency to vary innate in 

 groups of organisms. Variation is never spontaneous. 



Even in the quasi-spontaneous cases of mutation, we have 

 good reason to assmne a cause outside the organism. Variation 

 is caused by crossing. Closed groups lose their variability au- 

 tomatically, and we need not invoke nattiral selection to ac- 

 count for the eventual purity of the species. 



In practical plant and animal-breeding our modern biome- 

 chanic conception of heredity as the transmission of genes, 

 which may be factors in the development of both parents and 

 off-spring, has not contributed much of a positive nature. 



The best we can say for it is, that we have been able to prove 

 to our satisfaction why certain methods of selection have been 

 more effective than others. In most instances the breeders have 

 empirically found the right way, and it is only rarely, that we 

 Geneticians can find a case, in which we are able to point the 

 way to the breeders. 



Practically everywhere, the plant-breeders have abandoned 

 their selection according to individual merit, which is selection 

 according to phenotj^e, and resorted to some system of judg- 

 ing plants after their progeny. The whole elaborate system of 

 selection of sugar-beets amounts to a comparison of the prog- 

 eny of different plants, with a system of checking designed to 



