43i
But this is not all. For when Mr. Darwin acknowledges
that he has not paid sufficient attention to the influence of
changed conditions, what does he add ?—
“ In my opinion the greatest error which I have committed has
been not allowing sufficient weight to the direct action of the
environment—i.e., food, climate, &c.—independently of Natural
Selection. Modifications thus caused, which are neither of advantage
nor disadvantage to the modified organism, would be especially
favoured, as I can now see, chiefly through your observations, by
isolation in a small area, where only a few individuals lived under
nearly uniform conditions.”—{Life and Letters, vol. Hi., ft. /jp.)
And yet his opinion upon this subject seems to be far
from fixed, for elsewhere he says :—
“In North America, in going from north to south, or from east to
west, it is clear that the changed conditions of life have modified the
organisms in the different regions, so that they now form distinct
races, or even species. It is further clear that in isolated districts,
however small, the inhabitants almost always get slightly modified,
and how far this is due to the nature of the slightly different
conditions to which they are exposed, and how far to mere inter
breeding in the manner explained by Weismann, I can form no
opinion.”—{Life a?id Letters, vol. Hi., ft. 161.)
But interbreeding is equivalent to panmixia, and panmixia
is equivalent to the absence of Natural Selection. What,
then, is the opinion of Mr. Darwin on this subject?
1. That in a limited area Natural Selection will act, or
will tend to act, through the uniformity of the conditions.
2. That Natural Selection will tend not to act through the
small output of variation which there necessarily must be
in a small area. 3. That transforming influence—not
selection—will act in a limited area, of which the case of
the Saturnia is held to be a proof. 4. He does not know
whether to attribute the transmutation to transforming
influence or to interbreeding, i.e., the absence of Natural
Selection.
The phenomena which we have been considering seem
to accord with the idea of a transforming influence ; they