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,
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. 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 "a c" or "a d," the difference would be so great as to constitute distinct genera. Cope and 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 into 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.
(255/1. After speaking of Cope's comparison of acceleration and retardation in evolution to the force of gravity in physical matters Mr. Hyatt 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.
LETTER 256. TO A. HYATT. Down, December 14th .
(256/1. In reply to the above letter (255) from Mr. Hyatt.)
Notwithstanding the kind consideration shown in your last sentence, I must thank you for your interesting and clearly expressed letter. I have directed my publisher to send you a copy of the last edition of the "Origin," and you can, if you like, paste in the "From the Author" on next page. In relation to yours and Professor Cope's view on "acceleration" causing a development of new characters, it would, I think, be well if you were to compare the decapods which pass and do not pass through the Zoea stage, and the one group which does (according to Fritz Muller) pass through to the still earlier Nauplius stages, and see if they present any marked differences. You will, I believe, find that this is not the case. I wish it were, for I have often been perplexed at the omission of embryonic stages as well as the acquirement of peculiar stages appearing to produce no special result in the mature form.