By this post I return you Häckel’s essay on Perigenesis.² Although I have kept it so long, I have only just read it, as you said there was no need to return it at any particular time.

To me it seems that whatever merit Häckel’s views may have in this matter, they certainly have no claim to be regarded as original; for I cannot see that his ‘Plastidules’ differ in anything but in name from Spencer’s ‘Physiological Units.’³ Why he does not acknowledge this, it is difficult to understand. Anyhow, the theories being the same, the same objections apply; and to me it has always seemed that this theory is unsatisfactory because so general. As you observe in your letter, everyone believes in molecular movements of some kind; but to offer this as a full explanation of heredity seems to me like saying that the cause, say, of an obscure disease like diabetes, is the persistence of force. No doubt this is the ultimate cause, but the pathologist requires some more proximate causes if his science is to be of any value. Similarly, I do not see that biology gains anything by a theory which is really but little better than a restatement of the mystery of heredity in terms of the highest abstraction. Pangenesis at least has the merit of supplying us with some conceivable carriers, so to speak, of the modified protoplasm from the various organs or parts of the parent to the corresponding organs or parts of the offspring, and the multiplication of gemmules seems to me to avoid a difficulty with which Perigenesis (as stated by Häckel) is beset, viz. that atavism sometimes occurs over too large a gap to be reasonably attributed to what remains of the original ‘stem-vibrations’ after their characters have been successively modified at each ‘bifurcation.’⁴ But it would be tedious to enter into details. Perigenesis, in my opinion, is ‘more simple’ than Pangenesis, only because its terms are so much more general.

P.S.— I forgot to tell you, when we were at lunch, that the seed of the grafted beets is ready for sowing; also that the vine is now four feet high, and so, I should think, might be grafted next spring.⁵