After long reflection I cannot avoid the conviction that no innate tendency to progressive development exists, as is now held by so many able naturalists, and perhaps by yourself. It is curious how seldom writers define what they mean by progressive development; but this is a point which I have briefly discussed in the "Origin." I earnestly hope that you may visit Hilgendorf's famous deposit. Have you seen Weismann's pamphlet "Einfluss der Isolirung," Leipzig, 1872? He makes splendid use of Hilgendorf's admirable observations. (254/2. Hilgendorf, "Monatsb. K. Akad." Berlin, 1866. For a semi-popular account of Hilgendorf's and Hyatt's work on this subject, see Romanes' "Darwin and after Darwin," I., page 201.) I have no strength to spare, being much out of health; otherwise I would have endeavoured to have made this letter better worth sending. I most sincerely wish you success in your valuable and difficult researches.
I have received, and thank you, for your three pamphlets. As far as I can judge, your views seem very probable; but what a fearfully intricate subject is this of the succession of ammonites. (254/3. See various papers in the publications of the "Boston Soc. Nat. Hist." and in the "Bulletin of the Harvard Museum of Comp. Zoology.")
LETTER 255. A. HYATT TO CHARLES DARWIN. Cannstadt bei Stuttgart, December 8th, 1872.
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. 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. 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,
a--ab--abc--ae --------ad,
which like "a d" comes from the earliest of a series, though I should think from the entire skipping of the Zoea stage that it must be, like "a e," 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] 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, and which, I am sorry to see, has cost you some of your valuable time. 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.