VARIATION 119 



not know how the variations have been brought about, 

 though he can often reason from analogy. Among 

 birds and mammals, especially, it has become custom- 

 ary to recognize subspecies. A subspecies is a phase or 

 form which is reasonably true to type within a given 

 area, but at one or more points intergrades with its 

 allies occupying adjacent territory. As Beebe has 

 shown in the case of birds, the peculiarity (e.g., a 

 darker or lighter color) may be due to the immediate 

 effects of environment, and the intergradation may be 

 merely the expression of the intergrading climatic condi- 

 tions. On the other hand, Sumner, experimenting with 

 subspecies of wild mice, has found genuine hereditary 

 differences. Mere inspection would not show which 

 kind of " subspecies" we were dealing with. Suppose 

 the differences to be inherited, the intergradation where 

 two types meet may be due to hybridization. Ento- 

 mologists recognize varieties and aberrations. The 

 aberration or "sport" is supposed to occur occasionally, 

 here and there. It may be known only by a single 

 specimen, though the species to which it belongs is 

 common. It is found, however, that the same kind of 

 difference may distinguish an aberration in one place, 

 and a local race or subspecies in another ; and exactly 

 the same thing is true of plants. Botanists use the 

 word "form" or "forma" to designate minor varieties, 

 but with no regard to their genetic significance. Ulti- 

 mately the nomenclature of varieties will have to be 

 revised in the light of genetic research, but it is not 

 possible to do this thoroughly at present. 



