THROUGH CUMULATIVE SEGREGATION. 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 stock many that are equally fitted? ” (p. 197). 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 difference 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 by the separation 
betiveen the castes of India, or betiveen different qenera 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 
ivithout 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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