SELECTION. 187 



which are potentially new species, must be an altogether com- 

 mon occurrence. 



What we call natural selection is after all nothing but this 

 process,, the final outcome of such experiments in species-for- 

 mation. In almost every instance the experiment fails. A 

 small colony of individuals may have a potential variability 

 which includes the possibility of a new genotj^e with a corres- 

 ponding new character (or without), but eventually iJUrity 

 may be reached for the geno-combination of the parent-species, 

 in which case this colonization differs in nothing from ordinary 

 colonization. In other instances the group actually attains to 

 a distinct genotype, but the causes, which brought about the 

 original colonization, later on bring large numbers of the par- 

 ent-species into contact with the group, which is accordingly 

 swamped. 



It is also easily seen, how circumstances during the first 

 few generations favour a certain type, which accordingly be- 

 comes the tj^e of the new species, whereas this group, which 

 now has lost its plasticity, cannot continue to exist through- 

 out the range of variation in the conditions of life through 

 many generations. So that finally we see, that the chances for 

 the succesful estabhshment of a new species narrow down 

 considerably. For a new species to become estabhshed, a ser- 

 ies of conditions must be fulfilled. 



In the first place a group can only give rise to new species, 

 if the range of genotypic possibihties is not exhausted in the 

 material on hand, groups of aninjaJs and plants have been 

 brought into cultivation for instance, which possessed no Po- 

 tential variabihty, and from which therefore no new domestic 

 species can be derived, such as the guinea-fowl and the Reeve's 

 pheasant. Cross-breeding between geno-typically different 

 groups is a first essential for evolution of species. For the pro- ' 

 duction of varieties this requirement suffices, heightening of 

 the Potential variability of any group, any species, necessarily 

 leads to the production of varieties. But for the production of 

 species, and for thechange of varietiesintospecies more is needed. 



