SPECIES IN THE MAKING 113 



of the polar ice-cap — a necessary feature of the physical 

 conditions of the terrestrial globe. But it is the fashion 

 with a certain school of writers nowadays to declare 

 that " variation '' in organisms is a " mystery " unsolved. 

 Another very common and almost universal error is to 

 overlook the fact that variation is constitutional and 

 affects whole systems of organs and their deeply related 

 parts, and is not, as it is so frequently and erroneously 

 assumed to be, a mere local affair of patches and scraps 

 visible on this or that part of the surface of an animal or 

 plant. These superficial " marks," readily seen and noted 

 by the collector, are rarely of any life-saving importance : 

 they are but the outward and visible signs of deep-lying 

 physiological or constitutional change or variation. The 

 varying organism has, like Hamlet, "that within which 

 passeth show" and the superficial variations (like his 

 " inky cloak " and other customary features of mourning) 

 are but " the trappings and the suits " of a deep-lying 

 change. Variation is not an inexplicable mystery, nor, 

 on the other hand, are " varieties " sufficiently dealt with 

 and their nature appreciated when one or two surface 

 peculiarities are enumerated by which the collector can 

 recognize them. A deeper study of the varying organism 

 is both possible and needed. 



If the gradual formation of new species from ancestral 

 species is a true account of the matter, we must expect to 

 find, at any rate here and there, if not frequently, traces of 

 the process — for instance, gradations, or series of inter- 

 mediate forms, connecting new, well-established species 

 with the ancestral form or with one another. We do find 

 such gradations — sometimes more, sometimes less, com- 

 pletely persisting over a wide tract of country, or discover- 

 able in the fossiliferous deposits containing the remains of 

 extinct animals. 



For instance, when we look at the butterflies of a much 

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