VARIATION AND LOCALITY 131 



First, that when the details of the geographical distribution of 

 any variable species are studied in that thorough and minute 

 fashion which is necessary for any true knowledge of the inter- 

 relations of the several forms, the conception of a species invented 

 by the popular expositions of Evolution under Selection is found 

 to be rarely if ever realised in nature. 



A species in this generalised sense is an aggregate of indi- 

 viduals, none exactly alike, but varying round a normal type, 

 the characters of which are fixed in so far as they are adapted to 

 environmental exigency. In nature, however, the occurrence of 

 the varieties, and even the occurrence of the variability is 

 sporadic. In one place a population may be perfectly uniform. 

 In another it may be again uniform but distinct. In others 

 the two forms may occur together, sometimes with and sometimes 

 without intergrades. In some localities a sporadic variety 

 may be an element of the population, persisting through long 

 periods of time. In other localities there may be several such 

 aberrations occurring together which are absent elsewhere. 

 Secondly, I would remind the reader that in the light of genetic 

 analysis we know that intergrades, when they do occur, cannot 

 be assumed to represent conditions through which the species 

 must pass or has passed on its way to the extreme and definite 

 forms. 



Often, perhaps generally, they are nothing but heterozygous 

 forms, and often also they are conditions corresponding with the 

 presence of factors in their reduction-stages. 



A broad survey of the facts shows beyond question that it 

 is impossible to reconcile the mode of distribution of local forms 

 with any belief that they are on the whole adaptational. Their 

 peculiarities are occasionally the result of direct environmental 

 influence, as we shall hereafter notice in certain cases, but none 

 can attribute such sporadic and irregular phenomena to causes 

 uniformly acting. 



Writers on systematics, especially those of former generations 

 often conjecture or assert that local distinctions are caused by 

 "differences of climate, soil, food, etc.," in vague general terms. 

 It is usually safe to assume that these remarks do not represent 



