6 



each other, regularly breeding together within the species only and pro- 

 ducing like species as offspring. Distinct species do not commonly inter- 

 breed, but, when they do so, they form crosses or hybrids that are usually 

 sterile. Up to comparatively recent years no smaller division was recog- 

 nized, but with intensive study of material it has become evident to 

 advanced students that within the species there is considerable individual 

 and geographical variation. 



Individual variation is the natural difference that may occur at any 

 time between members of common parentage such as amongst full brothers 

 and sisters. Just as like begets like, so within certain limits like begets 

 unlike, for no two creatures are ever exact duplicates. This individual 

 variation, usually small and irregular in appearance and direction, but 

 sometimes persisting progressively generation after generation in one direc- 

 tion, forms the successive steps by which present day evolutionisms explain 

 the origin of new species. Individual variation, however, is disregarded in 

 classification unless it has proceeded far enough to produce marked 

 and constant differentiation over a definable natural group of a species. 



Geographical variation can be regarded as the result of a common 

 tendency to individual variation acting over a whole community of indi- 

 viduals tending towards a common goal and is held to be induced and 

 directed by local climatic and other conditions. Thus we often find that 

 within a widespread species all individuals inhabiting certain localities 

 have characteristics that separate them from those of the surrounding areas. 

 Individuals in a dry desert country are apt to be smaller and lighter in 

 coloration, whereas those in a warm, moist country are usually larger and 

 darker. These differences are sometimes marked and obvious; at other 

 times they are so slight as to be noticeable only by comparing large 

 numbers of specimens and can be detected only by averages. Thus there 

 is every degree of differentiation, due to geographical habitat, from pro- 

 nounced departures from type, of almost specific value, to the finest shades 

 of differentiation that skilled specialists can distinguish and which are 

 inappreciable to the ordinary eye. The outstanding fac', however, that 

 prevents the most marked geographical variation from full specific standing 

 is that these minor forms intergrade and in intermediate localities every 

 shade of differentiation between the extremes can be found. Between 

 species this gradual merging of character is not supposed to occur, and 

 however fine the distinctions may be, the divisions should be sharp and 

 defined. We, therefore, recognize these intergrading variations due to or 

 based upon geographical distribution as Geographical Races, Varieties, or 

 Subspecies, the last term being now the most usual, and we regard them 

 as species in the making before the connecting stages binding them to the 

 original stock have disappeared, owing to the increasing sterility between 

 the extreme variants. Except in such rare cases of physical isolation, 

 as where an oceanic island habitat precludes continuous distribution, 

 we take, in practice, the existence of intergrades as the evidence of sub- 

 specific status. Besides these divisions of taxonomic value there are a 

 few other variants that, owing to their erratic occurrence, cannot be 

 recognized in our classification. These are "Albinos," "Melanos," and 

 "Dichromatic Forms." 



