228 EVOLUTION IN NATURE AND UNDER DOMESTICATION. 



ent species, heightens the variability of that species into which 

 the hybrids merge, but the effect must be only temporary, and 

 no change in the type of the species will result. 



We cannot but conclude, that in different cases the course of 

 evolution must be a different one. Still, it looks as if in the ma- 

 jority of instances species are pure for their type, and do not 

 change by continued natural selection, although this selection 

 may have had a marked influence when these species origin- 

 ated. Occasional crossings (between such species as are called 

 sub-species) heighten the total variability of species, and this 

 variability is continually reduced automatically. Wherever it 

 exists, even if it has no influence on the type of the species 

 itself, it makes it possible for a split-off group, an isolated small 

 colony to have a total potential variability different from that 

 of the old species, and therefore, under the influence of natural 

 selection automatically to become pure for its own specific 

 type, its own genotype. 



Colonization must be a common phenomenen, and it seems, 

 as if the directive action of natural selection on species were 

 restricted to the very short period during which an islolated 

 group becomes pure, becomes a species. 



To become established as a new species, new sub-species, a 

 group of organisms must be isolated, either in space or physio- 

 logically, for a time, sufficiently long for the group to become 

 numerically important enough to be able to withstand occasion- 

 al contact with the species from which it diverged. 



If the isolation is a mere isolation in space, a colonization, 

 those new species will stand the best chance to survive, which 

 are as well or better fitted to live than the parent-species. A 

 new species cannot survive if there is not some reason, which 

 makes matings between individuals belonging to it much more 

 frequent than inter-specific matings. 



The very fact, that there are common and rare species, testi- 

 fies against an effect of natural selection upon established spe- 

 cies. Those species are common whose genotype makes them 

 fitted to live and procreate. We can roughly distinguish two 



