DIVERGENT EVOLUTION THROUGH SEGREGATION. PAPA 
uted 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 barriers, or even the occupation of separate 
districts. This may be illustrated by the separation between the castes of 
India, or between different genera occupying the same locality. - - - 
We must suppose that they [the diverging forms] must possess an in- 
herent 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 generations, 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 con- 
dition sufficient 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 varia- 
tion depend chiefly upon the nature of the organism; and that while 
the nature of the external conditions has power to winnow out what- 
ever forms are least fitted to survive, there will usually remain a number 
of varieties equally fitted to survive; and that through the law of segre- 
gation constantly operating in species distributed over considerable 
areas, these varieties continue to diverge both in form and in habits till 
separate species are fully established, though the conditions are the 
same throughout the whole area occupied by the diverging forms.” 
The conclusion reached was that ‘‘ The theory that diversity of natu- 
ral selection is, like variation, an essential factor in producing diversity 
of species, is untenable. On the contrary, we find that diversity of 
natural selection is not necessary to diversity of evolution, nor uni- 
formity of natural selection to uniformity of evolution; but while 
variation and separation are the essential factors in diversity, and inter- 
crossing and unity of descent the essential agents in uniformity of 
evolution, natural selection may be an important ally on either side.” 
In an article on * Evolution in the Organic World,” published in 
