246 EVOLUTION IN NATURE AND UNDER DOMESTICATION. 



riers do not exist and where local species gradually merge one 

 in to the other, we meet a state of things different from that in 

 which local species exist on both sides of a region not suitable 

 for occupation for these organisms, or where local species fit 

 different ecological niches in the same locality. In the latter 

 cases the species differ in groups of genes, whereas in the ab- 

 sence of barriers the gradual change of type which we observe 

 in passing over one continuous region, is due to the fact that 

 differentiating genes are present in groups of organisms, which 

 groups do not correspond for the different genes. Each differ- 

 entiating gene, by which we mean each gene which is present 

 only in part of the organisms of the whole group, has a certain 

 territory. Whenever we are dealing with a gene, which has 

 such a marked influence upon the characters of organisms car- 

 rying it, that the difference caused by its presence or absence 

 is greater than any chance difference through environmental 

 influences, we note discontinuous variation in some character 

 in addition to continuous variation in a number of other char- 

 acters. But where we are dealing with genes whose influence 

 is less, or with several genes which influence the same char- 

 acter in the same or in opposite ways, the differences produced 

 by their presence and absence are gradual. 



It is extremely difficult to decide which groups in such series 

 of organisms should be called species. The safest way, we 

 think, is to give specific names to as many types as can be dis- 

 tinguished. These species with a limited range and no very 

 sharp boundaries are certainly not varieties. 



Whenever we find two or more species of obviously very 

 closely related species living in the same territory, we know 

 that something must interfere with their free inter-crossing. If 

 autogamy is excluded we will find that in most cases the two 

 species live a somewhat different life, they live in the same 

 geographic region, but in different spots, they have habits 

 which bring members of the same species into very much closer 

 contact than members of different species. But sometimes 

 we find other reasons for the high proportion of intra-specif ic 



