More Letters of Charles Darwin — Volume 1 · Charles Darwin
LETTER 254. TO A. HYATT. Down, December 4th, 1872.
I thank you sincerely for your most interesting letter. You refer much too modestly to your own knowledge and judgment, as you are much better fitted to throw light on your own difficult problems than I am. It has quite annoyed me that I do not clearly understand yours and Prof. Cope's views (254/1. Prof. Cope's views may be gathered from his "Origin of the Fittest" 1887; in this book (page 41) is reprinted his "Origin of Genera" from the "Proc. Philadelph. Acad. Nat. Soc." 1868, which was published separately by the author in 1869, and which we believe to be his first publication on the subject. In the preface to the "Origin of the Fittest," page vi, he sums up the chief points in the "Origin of Genera" under seven heads, of which the following are the most important:--"First, that development of new characters has been accomplished by an ACCELERATION or RETARDATION in the growth of the parts changed...Second, that of EXACT PARALLELISM between the adult of one individual or set of individuals, and a transitional stage of one or more other individuals. This doctrine is distinct from that of an exact parallelism, which had already been stated by von Baer." The last point is less definitely stated by Hyatt in his letter of December 4th, 1872. "I am thus perpetually led to look upon a series very much as upon an individual, and think that I have found that in many instances these afford parallel changes." See also "Lamarck the Founder of Evolution, by A.S.
On the other hand the skulls of some mammals go on altering during maturity into advancing years; as do the horns of the stag, the tail-feathers of some birds, the size of fishes etc.; and all such differences I should attribute simply to the laws of growth, as long as full vigour was retained. Endless other changes of structure in successive species may, I believe, be accounted for by various complex laws of growth. Now, any change of character thus induced with advancing years in the individual might easily be inherited at an earlier age than that at which it first supervened, and thus become characteristic of the mature species; or again, such changes would be apt to follow from variation, independently of inheritance, under proper conditions. Therefore I should expect that characters of this kind would often appear in later-formed species without the aid of Natural Selection, or with its aid if the characters were of any advantage. The longer I live, the more I become convinced how ignorant we are of the extent to which all sorts of structures are serviceable to each species. But that characters supervening during maturity in one species should appear so regularly, as you state to be the case, in succeeding species, seems to me very surprising and inexplicable.
With respect to degradation in species towards the close of a series, I have nothing to say, except that before I arrived at the end of your letter, it occurred to me that the earlier and simpler ammonites must have been well adapted to their conditions, and that when the species were verging towards extinction (owing probably to the presence of some more successful competitors) they would naturally become re-adapted to simpler conditions. Before I had read your final remarks I thought also that unfavourable conditions might cause, through the law of growth, aided perhaps by reversion, degradation of character. No doubt many new laws remain to be discovered. Permit me to add that I have never been so foolish as to imagine that I have succeeded in doing more than to lay down some of the broad outlines of the origin of species. 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.
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.
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