MUSEUM OF COMPARATIVE ZOOLOGY. 119 



power of climatic influence, seems to furnish a far more satisfactory 

 explanation of these perplexing phenomena. But an advocate of the 

 theory of hybridity might still assume that this gradual transition over a 

 wide area is no objection to the theory, since the gradual fading out of 

 the impression of contact in either direction from the line of junction of 

 the respective habitats of two forms is just the result that would be 

 anticipated from such a sexual intermingling of the forms in question. 

 But the real objection to the theory — granting the possibility- of hy- 

 bridization on such a gigantic scale, which seems really improbable — 

 is, that widely different forms occur also at different points in latitude, 

 between which each successive stage of gradual differentiation can be 

 readily traced, where hybridity can scarcely be supposed to account for 

 the gradual change. Furthermore, a differentiation is now known in 

 so many cases that it amounts to the demonstration of climatic varia- 

 tion as a general law, by means of which a species may be safely pre- 

 dicted to take on a given character under certain specific climatic 

 conditions. If the theory of hybridity be urged to account for the in- 

 tergradation of forms occurring at localities differently situated in respect 

 to latitude, as has been sometimes done, it evidently falls under the 

 weight it has to support; and yet there seems to be little better evi- 

 dence in its hehalf in cases where the intcrgrading forms happen to he 

 differently situated in respect to longitude. 



In regard to how these well-marked geographical forms shall be rec- 

 ognized, there may he just grounds for a diversity of opinion. Evi- 

 dently in cases where they are slightly marked or somewhat inconstant, 

 no great harm would result if they were nominally ignored. Practi- 

 cally, most naturalists recognize as species such groups of individuals as 

 are not known to graduate by nearly imperceptible stages into any 

 other similar group ; and as varieties, such groups of individuals as 

 occur at certain localities, or over certain area-, which differ more or less 

 from other groups of individuals inhabiting other (general!)' contiguous) 

 localities, with which there is evidence that they do, more or less fully, 

 intergrade. Convenience seems to demand such a course, in order to 

 enable the naturalist to specify what particular phase or race of a spe- 

 cies inhabits a given section of country ; the first specific name used for 

 any part of the group being, of course, retained for the longest known 

 form, and the other races, when of such prominence as to rend i naming 

 them advisable, being designated by additional varietal names; as, tor 



