No. 636] SHORTER ARTICLES AND DISCUSSION 85 



not possessed by the others. Along the geographic borders of 

 each subspecies will be found specimens showing varying de- 

 grees of intergradation so that each form merges with an ad- 

 joining one, or, in some cases, one in central position may merge 

 into several others in different directions. If, for any reason, 

 the "areas of intergradation" were rendered uninhabitable, the 

 various subspecies would stand as distinct well-characterized 

 forms presumably until they themselves began to differentiate 

 and separate into parts. Sometimes the difference between rec- 

 ognizable subspecies is slight, or sometimes it is very marked, 

 but when gradations through several forms are followed, char- 

 acters are almost always found to change to a degree far be- 

 yond any probability of an ontogenetic explanation. It may be 

 emphasized that subspecies of this sort are not the exception, but 

 the rule. It might almost be said that the existence of diverse 

 inosculating units correlated with geography is characteristic of 

 terrestrial vertebrates. Continued study with improved facili- 

 ties and increasingly comprehensive collections from all parts 

 of the world constantly reduces the number of forms which 

 are not known to break up into subspecies. To a very great 

 extent, the presence of an undivided 'full species" in our check- 

 lists signifies either that it is a senescent type of limited distri- 

 bution or that, for lack of material or opportunity, it has not 

 been studied intensively. The intergrading subspecies has not 

 been recognized so widely among invertebrates nor in plants, but 

 neither entomologists nor botanists have collected and studied 

 their material from the geographic standpoint to such an extent 

 as the ornithologists and mammalogists, so it cannot be said that 

 the process of change illustrated by the subspecies is not even 

 more widespread than appears from the study of birds and- 



The process of formation of these subspecies, therefore, is 

 going on before our very eyes in wholesale fashion and it is diffi- 

 cult to believe that it is, as someone has said, merely a "shuffling- 

 of the cards" wliich in the long run means nothing to evolution- 

 ary progress. Rather does it seem that it must have a physio- 

 logical basis, a relation to germinal change, and a large poten- 

 tiality for affecting the general course of evolution. Despite its- 

 evident importance, the intergrading subspecies is receiving but. 

 scant attention from experimental zoologists. With the con- 

 spieious exception of the very significant work being done with 



