Polymorphic Species 69 



Sports and spontaneous variations must now be considered. It 

 is well known that they have produced a large number of fine 

 horticultural varieties. The cut-leaved maple and many other trees 

 and shrubs with split leaves are known to have been produced 

 at a single step ; this is true in the case of the single-leaf strawberry 

 plant and of the laciniate variety of the greater celandine : many 

 white flowers, white or yellow berries and numerous other forms 

 had a similar origin. But changes such as these do not come under 

 the head of adaptations, as they consist for the most part in the loss 

 of some quality or organ belonging to the species from which they were 

 derived. Darwin thinks it impossible to attribute to this cause the 

 innumerable structures, which are so well adapted to the habits of life 

 of each species. At the present time we should say that such adapta- 

 tions require progressive modifications, which are additions to the 

 stock of qualities already possessed by the ancestors, and cannot, 

 therefore, be explained on the ground of a supposed analogy with 

 sports, which are for the most part of a retrogressive nature. 



Excluding all these more or less sudden changes, there remains 

 a long series of gradations of variability, but all of these are not 

 assumed by Darwin to be equally fit for the production of new 

 species. In the first place, he disregards all mere temporary varia- 

 tions, such as size, albinism, etc.; further, he points out that very 

 many species have almost certainly been produced by steps, not 

 greater, and probably not very much smaller, than those separating 

 closely related varieties. For varieties are only small species. Next 

 comes the question of polymorphic species : their occurrence seems to 

 have been a source of much doubt and difficulty in Darwin's mind, 

 although at present it forms one of the main supports of the pre- 

 vailing explanation of the origin of new species. Darwin simply states 

 that this kind of variability seems to be of a peculiar nature ; since 

 polymorphic species are now in a stable condition their occurrence 

 gives no clue as to the mode of origin of new species. Polymorphic 

 species are the expression of the result of previous variability acting 

 on a large scale ; but they now simply consist of more or less numerous 

 elementary species, which, as far as we know, do not at present exhibit 

 a larger degree of variability than any other more uniform species. 

 The vernal whitlow-grass (Draba vernci) and the wild pansy are the 

 best known examples; both have spread over almost the whole of 

 Europe and are split up into hundreds of elementary forms. These 

 sub-species show no signs of any extraordinary degree of variability, 

 when cultivated under conditions necessary for the exclusion of inter- 

 crossing. Hooker has shown, in the case of some ferns distributed 

 over still wider areas, that the extinction of some of the intermediate 

 forms in such groups would suffice to justify the elevation of the 



