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Darwin Correspondence Project

From Alpheus Hyatt   8 December 1872

Cannstadtt bei Stuttgart

Dec. 8th, 1872

Charles Darwin

Dear Sir

The quickness and earnestness of your reply to my letter gives me the greatest encouragement, and I am much delighted at the unexpected interest, which your questions and comments display. What you say about Prof. Cope’s style has been often before said to me, and I have remarked in his writings an unsatisfactory treatment of our common theory.1 This, I think, perhaps is largely due to the complete absorption of his mind in the contemplation of his subject, this seems to lead him to be careless about the methods in which it may be best explained. He has, however, a more extended knowledge than I have, and has in many ways a more powerful grasp of the subject, and, for that very reason perhaps is liable to run into extremes. You ask about the skipping of the Zoea stage in fresh-water decapods, is this an illustration of acceleration? It most assuredly is, if acceleration means anything at all.2 Again, another and more general illustration would be, if, among the Marine decapods, a series could be formed in which the Zoea stage became less and less important in the development and was relegated to younger and younger stages of the development, and finally disappeared in those to which you refer. This is the usual way in which the accelerated mode of development manifests itself, though near the lowest or earliest occurring species it is also to be looked for. Perhaps this to which you allude is an illustration somewhat similar to the one which I have spoken of in my series



which like “ad” comes from the earliest of a series though I should think from the entire skipping of the Zoea stage that it must be like “ae” the result of a long line of ancestors. In fact the essential point of our theory is, that characteristics are ever inherited by the young at earlier periods than they are assumed in due course of growth by the parents, and that this must eventually lead to the extinction or skipping of these characteristics altogether.

Such considerations as these and the fact that near the heads of series or near the latest members of series, and not at the beginning were usually found the accelerated types, which skipped lower characteristics and developed very suddenly to a higher and more complex standpoint in structure, led both Cope and [myself]3 into what may be a great error. I see that it has led you at least into the difficulty of which you very rightly complain, & which I am sorry to see has cost you some of your valuable time.4 We presumed that because characteristics were perpetually inherited at earlier stages, that this very concentration of the developed characteristics made room for the production of differences in the adult descendants of any given pair. Further that in the room thus made other different characteristics must be produced, and that these would necessarily appear earlier in proportion as the species was more or less accelerated and be greater or less in the same proportion. Finally that in the most accelerated such as “ac” or “ad” the difference would be (sheet 3) so great as to constitute distinct genera.5 Cope & I have differed very much, while he acknowledged the action of the accumulated mode of development only when generic characteristics or greater differences were produced, I saw the same mode of development to be applicable in all cases and to all characteristics, even to diseases. So far the facts bore us out, but when we assumed that the adult differences were the result of the accelerated mode of development, we were perhaps upon rather insecure ground. It is evidently this assumption which has led you to misunderstand the theory. Cope founded his belief, that the adult characteristics were also the result of acceleration, if I rightly remember it mainly upon the class of facts spoken of above in man where a sudden change in two organs may produce entirely new and unexpected differences in the whole organisation, and upon the changes which acceleration appeared to produce in the development of each succeeding species. Your difficulty in understanding the theory and the observations you have made show me at once what my own difficulties have been, but of these I will not speak at present as my letter is spinning itself out to a fearful length

[After speaking of Cope’s comparison of acceleration & retardn in evolution to the force of gravity in physical matters he goes on:—]

Now it [acceleration] seems to me to explain less and less the origin of adult progressive characteristics or simply differences, and perhaps now I shall get on faster with my work.


See letter to Alpheus Hyatt, 4 December [1872] and n. 3. Hyatt refers to Edward Drinker Cope.
All the square brackets in the letter are in the copy from which this transcription is taken.
In his letter to Hyatt of 4 December [1872], CD wrote that one reason for his misunderstanding of acceleration was that he wrongly assumed it was the cause of the differences between successive species.
The notion that acceleration made room for the production of new differences in successive generations was central to Cope’s theory of the ‘origin of genera’, which posited a largely non-adaptive explanation of the course of evolution (see Cope 1868).


Cope, Edward Drinker. 1868. On the origin of genera. Proceedings of the Academy of Natural Sciences of Philadelphia (1868): 242–300.


Discusses his theory of acceleration and retardation of development.

Letter details

Letter no.
Alpheus Hyatt
Charles Robert Darwin
Sent from
Cannstatt bei Stuttgart
Source of text
DAR 145: 365
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 8665,” accessed on 26 October 2021,

Also published in The Correspondence of Charles Darwin, vol. 20