From Herbert Spencer¹   22 February 1860

You have wrought a considerable modificⁿ in the views I held— While having the same general conception of the relation of species genera, orders &c as gradually arising by differentiation & divergence like the branches of a tree & while regarding these cumulative modifications as wholly due to the influence of surrounding circums^s. I was under the erroneous impression that the sole cause was adaptation to changing conditions of existence brought about by habit, using the phrase conditions of existence in its widest sense as including climate, food, & contact with other organisms (for general statement of this view see Essay pp. 41. 45.)² But you have convinced me that throughout a great proportion of cases, direct adaptation does not explain the facts, but that they are explained only by adaptation through Natural Selection—

Many (&c) must have been struck with the fact that among all races of organisms the tendency was for the best individ^s. only to survive & that so the goodness of the race was preserved

I have in Essay on Population &c remarked this as a cause of improvement among mankind—³

But I & every one overlooked the selection of “spontaneous” variations without which I think you have clearly shown that many of the phenomena are insoluble.

You have shown that the doctrine furnishes explanations to phenomena otherwise inexplicable. However the argument may as yet fall short of direct demonstration yet the indirect demonstⁿ. is to me conclusive. I take it to be incredible that so many different kinds of evidence sh^d coincide in supporting a doctrine that was untrue—