76 COLORATION IN POLISTES. 



The local races of any given region, as compared collectively with those of con- 

 tinguous regions, and the manner of their mutual intergradations, point plainly 

 to some general or widely acting cause of differentiation. This is indicated by the 

 constancy of the results — so many species belonging to numerous and widely dis- 

 tinct groups being similarly affected. 



Allen further asks : 



Will the fortuitous, spontaneous results of natural selection yield a satisfactory 

 explanation of these phenomena, or must we seek some more uniform and defi- 

 nitely acting cause ? 



After an inquiry into the adequacy of natural selection in originating 

 characters of this nature, he states his conclusions as follows : 



While there is perhaps little reason to question the correctness of these generali- 

 zations [concerning natural selection], they have little bearing upon the question 

 of the modification of species by the direct action of climatic conditions, but relate 

 mainly to such unfavorable climatic influences as tend toward the extinction of 

 species, or to the circumscription of their ranges. Indeed, the phenomena of varia- 

 tion detailed in the foregoing pages were almost wholly unknown at the time the 

 earlier editions of the " Origin of Species " were published, and have hardly as yet 

 become the common property of naturalists. Gradual decrease in size southward 

 in hundreds of species inhabiting the same continent, or a gradual increase or 

 decrease in color in given directions on a similarly grand scale, are facts but recently 

 made known, and have not as yet been very fully discussed by evolutionists of the 

 purely Darwinian school. • • • • That varieties may and do arise by the action 

 of climatic influences, and pass on to become species, and that species become, in 

 like manner, differentiated into genera, is abundantly indicated by the facts of 

 geographical distribution and the obvious relation of local forms to the conditions 

 of environment. The present more or less unstable condition of the circumstances 

 surrounding organic beings, together with the known mutations of climate our 

 planet has undergone in past geological ages, points clearly to the agency of physical 

 conditions as one of the chief factors in the evolution of new forms of life. 



The establishment of local races exhibiting some peculiar type in the 

 general trends of divergence is to be explained on the principle of 

 cumulative segregation. Observations on the mating habits of the 

 species indicate that individuals belonging to the same colony tend to 

 interbreed. It is also probable that there is extensive mating outside 

 the colony ; but since individuals tend to range only over a limited terri- 

 tory, and further tend to occupy the same locality, and even the same 

 nesting site, year after year, we have here all the conditions which favor 

 the origination of nest individuality and local races. 



The same principle accounts for the maintenance of the distinction 

 between such a species as P. Q;alHca of Europe and its variety, diadenia. 

 Von Siebold has shown that even where the ranges of these two overlap 

 they occupy different stations, and consequently the changes which 

 different environmental conditions may effect are not lost by inter- 



