Chap. IV. 
DIVEEGENCE OF CHAEACTER. 
117 
that on an average more of the species of large genera 
vary than of small genera; and the varying species of 
,the large genera present a greater number of varieties. 
We have, also, seen that the species, which are the 
commonest and the most widely-diffused, vary more 
than rare species with restricted ranges. Let (A) be a 
common, widely-diffused, and varying species, belong¬ 
ing to a genus large in its own country. The little fan 
of diverging dotted lines of unequal lengths proceeding 
from (A), may represent its varying offspring. The 
variations are supposed to be extremely slight, but of 
the most diversified nature; they are not supposed all 
to appear simultaneously, but often after long intervals 
of time; nor are they all supposed to endure for equal 
periods. Only those variations which are in some way 
profitable will be preserved or naturally selected. And 
here the importance of the principle of benefit being 
derived from divergence of character comes in; for this 
will generally lead to the most different or divergent 
variations (represented by the outer dotted lines) being 
preserved and accumulated by natural selection. W^hen 
a dotted line reaches one of the horizontal lines, and is - 
there marked by a small numbered letter, a sufficient 
amount of variation is supposed to have been accu¬ 
mulated to have formed a fairly well-marked variety, 
such as would be thought worthy of record in a sys¬ 
tematic work. 
The intervals between the horizontal lines in the 
diagram, may represent each a thousand generations; 
but it would have been better if each had represented 
ten thousand generations. After a thousand genera¬ 
tions, species (A) is supposed to have produced two 
fairly well-marked varieties, namely a} and These 
two varieties will generally continue to be exposed to 
the same conditions which made their parents variable, 
