Undoubtedly your view is very important, as bearing on the general question of variability.

When I wrote the "Origin" I could not find any facts which proved the direct action of climate and other external conditions.

I long ago thought that the time would soon come when the causes of variation would be fully discussed, and no one has done so much as you in this important subject.

The recent evidence of the difference between birds of the same species in the N.
and S.United States well shows the power of climate.

The two sexes of some few birds are there differently modified by climate, and I have introduced this fact in the last edition of my "Descent of Man." (268/2.
"Descent of Man," Edition II.