240 EVOLUTION IN NATURE AND UNDER DOMESTICATION. 



variability will become pure. We picture the origin of species 

 in nature as brought about by any cause, isolating a group of 

 organisms from the swamping effect of free-crossing with the 

 species multitude, provided the total potential variability of 

 the isolated group permits of a new genotype, a new combin- 

 ation of genes. 



It is evident that any small group of individuals as such, 

 must have a potential variability smaller than that of the whole 

 species at the moment of isolation. Therefore it may seem 

 paradoxical to assume that under the influence of selection and 

 automatically, such a small isolated group can give rise to a 

 new species, whereas the old species remains unaltered. 



But it must be remembered that in such a small isolated 

 group, if it has any potential variability, that is to say, if it is 

 isolated at a moment when the total variability of the old spe- 

 cies has recently been heightened by a cross, the individuals of 

 the common specific type do not constitute a great preponder- 

 ant multitude. 



Whereas species in nature are stable, and do not change by 

 natural selection, it is easy to see how they can gradually be 

 replaced by new species, springing from them, reentering their 

 territory and which prove to be better fitted to live in the old 

 conditions of life. From paleontological evidence it is impossi- 

 ble to decide whether a species has changed as a whole, or 

 whether species have repeatedly crowded out their parent- 

 species. 



In cultivation, the main difference from what happens in 

 nature, is given by the control of propagation. There is no mul- 

 titude of geno-typically identical individuals into which 

 variants merge. Variants of some merit are selected, and contin- 

 ually a sort of colonization is taking place, starting with very 

 few individuals of a typical constitution. In cultivated animals 

 and plants the species can change as a whole. An extreme, but 

 very illustrative example is furnished by cattle. Here aberrant, 

 from the breeder's standpoint superior animals are not only 

 selected and mated to similar aberrant individuals, but very 



