From Herbert Spencer   8 February 1868

Dear Mr Darwin,

After lying for nearly a week at my publishers, to whom it had been sent in ignorance, I suppose, of my private address, your new work reached me a few days ago.¹ Thank you very much for it. I cannot help wishing that the immense mass of elaborated materials it contains, and all its subtile interpretations of special facts, had been accessible to me for these two or three years past; so that I might have been able to make use of them in illustration of my own arguments.²

I have at present done little more than dip here & there—paying more especial attention, however, to the speculation on “Pangenesis”, in which, I need hardly say, I am much interested.³ It is quite clear that you do not mean by “gemmules” what I mean by “physiological units”; and that, consequently, the interpretations of organic phenomena to which they lead you, are essentially different from those I have endeavoured to give.⁴ The extremely compound molecules (as much above those of albumen in complexity as those of albumen are above the simplest compounds) which I have called “physiological units,” and of which I conceive each ⁠⟨⁠orga⁠⟩⁠nism to have a modification peculiar to itself, I conceive to be within each organism substantially of one kind—the slight differences that exist amongst them, being such only as are due to the slight modifications of them inherited from parents and ancestry. The evolution of the organism into its special structure, I suppose to be due to the tendency of these excessively complex units to fall into that arrangement, as their form of equilibrium under the particular distribution of forces they are exposed to by the environment and by their mutual actions. On the other hand, your “gemmules,” if I understand rightly, are from the beginning heterogeneous—each organ ⁠⟨⁠    ⁠⟩⁠ the organism being the source of a different kind, and propagating itself, as a part of succeeding organisms, by means of the gemmules it gives off.⁵

I must try and throw aside my own hypothesis and think from your point of view, so as to see whether yours affords a better interpretation of the facts.

Sincerely yours | Herbert Spencer