THROTJGH CTJMTJLA.TIVE SEQEEGATION. 



191 



does the change expend itself in producing from each species just 

 one new species completely fitted to the conditions, or may it pro- 

 duce from one stocTc many that are equally fitted^ " (p. 497). In 

 answering these questions I called " attention to the variation 

 and distribution of terrestrial mollusks, more especially those 

 found on the Sandwich Islands," and gave what seemed to me 

 strong reasons for believing that ^^The evolution of these dif- 

 ferent forms cannot he attributed to diffei^ence in their external 

 conditions. ... If we would account for the difference and the 

 limited distribution of these allied forms on the hypothesis of 

 evolution from one original species, it seems to me necessary to 

 suppose two conditions, Separation and Variation. I regard 

 Separation as a condition of the species, and not of surrounding 

 nature, because it is a state of division in the stock which does 

 not necessarily imply any external harriers., or even the occupation 

 of separate districts. This may he illustrated hy the separation 

 hetween the castes of India, or hetween different genera occupying 

 the same locality. . . . We must suppose that they [the diverging 

 forms] must possess an inherent tendency to variation so strong 

 that all that is necessary to secure a divergence of types in the 

 descendants of one stock is to prevent, through a series of gener- 

 ations, their intermingling with each other to any great degree " 

 (pp. 498-499). I also called attention to the fact that some forms 

 of Natural Selection must " prevent variation and give a wider 

 diffusion to forms that would otherwise be limited in their range 

 and variable in their type. Natural Selection is as efficient in 

 producing permanence of type in some cases as in accelerating 

 variation in other cases " (p. 504). On page 499 I pointed out 

 the law that " The area occupied by any species must vary directly 

 as its power and opportunity for migration, and inversely as 

 its power of [divergent] variation." And on page 505 I gave a 

 brief summary of my reasons for believing that ^^Separation 

 without a difference of external circumstances is a condition suffi- 

 cient to ensure . . . divergence in type.^^ 



Subsequent investigation has led to the development of my 

 theory, with a fuller discussion of the causes and laws that are 

 revealed in these phenomena. In an article published in ' The 

 Chrysanthemum ' (Yokohama and London, Triibner & Co.), 

 January 1883, I state my belief " that the quality, the diversity, 

 and the rapidity of the variation depend chiefly upon the nature 

 of the organism ; and that while the nature of the external 



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