186 SELECTION. 



ent size, which prevents cross-breeding, if not absolutely, at 

 least to an extent which would be inconsistent with specific 

 identity. In the fields the field-rat exists together with a small 

 rat, which completely copies its habits and mode of life, and 

 \yhich therefore is probably not identical with the Mus con- 

 color which occurs in the houses. 



When a species becomes stable, and pure for its type, time 

 alone will decide whether it wiU be able to persist. It will only 

 persist as a species, if it is somehow protected from merging 

 into numerically larger groups, either by its geographic iso- 

 lation, or by any other kind of isolation, a structure of the 

 sexual organs producing self-fertilization, a great difference 

 in size, in time of flowering, or a constitution which makes its 

 members live a life which does not bring them into close con- 

 tact with their relatives. And on the other hand, it will only 

 survive if its individuals as such are well fitted to increase 

 their kind. 



In almost every natural order of plants and animals, new, 

 small groups, differing geno-ts^pically from the accepted con- 

 stitution of the established species must continually get into 

 coiiditions, where they are for the time being potential 

 species. 



There may exist species of animals or plants, reUcs of form- 

 er generations which are irrevocably shut-off from any chance 

 of a heightening of the potential variability, by the fact that all 

 their individuals have the same genotype, and that no spe- 

 cies happen to exist with which they produce fertile hybrids. 

 Ginkgo biloba may be in this condition. Generally speaking, 

 however, the chances for the production of groups, which have 

 such a potential variability, that they can eventually become 

 pure for a new genotype is open in every group of species or 

 sub-species, local species, within which cross-breeding is not 

 excluded. 



And as colonization is a normal course of reproduction in 

 the most diverse groups, in some habitually so, in others at 

 least occasionally, this trying out of munerically small groups 



