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The following is in answer to a letter that cannot be found:—

FROM DARWIN

Down, Beckenham, Kent,

June 1, 1871.

My dear Mr. Agassiz:

Very many thanks for your kind letter and curious facts about the fishes. What an extraordinary number of complex and wonderful structures have been developed in relation to sex!

I am also particularly glad to hear about the pedicellariæ of the Echinodermata, the homologies of which I did not in the least know. I must now find out the homologies of the "Birds-beaks" and serrated bristles of the Bryozoa, which I remember watching in old days with astonishment.

I am thinking of bringing out a new and cheap edition of the "Origin;" and if so I should give a chapter to answering, as far as I can and space permits, Mivart's very clever book. I have no doubt the book will produce a great effect on many; and you will think it blind prejudice when I say it has had none on me. There is not one new point in it, though many are admirably illustrated. Mivart never racks his brains to see what can be fairly said on (the) opposite side, and he argues as if I had said nothing about the effect of use or the direct action of external conditions: though in another part of his book on these points, almost every illustration is taken from my writings and observations.

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But I will not bother you with more remarks on this head.

Pray give my most sincere respects to your father. What a wonderful man he is to think of going round Cape Horn; if he does go, I wish he could go through the Strait of Magellan. With very kind remembrances from all of us to Mrs. Agassiz, and with many thanks from myself, Pray believe me, Yours very sincerely, Ch. Darwin.

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In August, 1880, Agassiz delivered an address before the American Association for the Advancement of Science, using his knowledge of the Echini to show the extreme difficulty, if not impossibility, of ever obtaining a complete record of the development of even a single group. In acknowledging the receipt of a reprint, Darwin writes [5 May 1881]:

"I read your address with much interest. However true your remarks on the genealogies of the several groups may be, I hope and believe that you have overestimated the difficulties to be encountered in the future. A few days after reading your address I interpreted to myself your remarks on one point (I hope in some degree correctly) in the following fashion:

"'Any character of an ancient generation or intermediate form may, and often does reappear in its de-

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scendants after countless generations, and this explains the extraordinary complicated affinities of existing groups.'

"This idea seems to me to throw a flood of light on the lines, sometimes used to represent affinities, which radiate in all directions often to very distant sub-groups — a difficulty which has haunted me for half a century. A strong case could be made out in favor of believing in such reversion or atavism after immense intervals of time. I wish the idea had been put into my head in old days, for I shall never again write on difficult subjects."

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To this Darwin replied in a letter characteristically full of courtesy and open-mindedness, qualities not always conspicuous in scientific discussions. It has already been published in "More Letters of Charles Darwin," but a few passages may not be out of place here: "You will have seen Mr. Murray's views on the formation of atolls and barrier reefs. Before publishing my book I thought long over the same view, but only as far as ordinary marine organisms are concerned, for at that time little was known of the multitude of min-

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ute oceanic organisms. I rejected this view as from the few dredgings made in the Beagle in the South Temperate regions, I concluded that shells, the smaller corals, etc., etc., decayed and were dissolved, when not protected by the deposition of sediment; and sediment could not accumulate in the open ocean.... I have expressly said that a bank at the proper depth would give rise to an atoll, which could not be distinguished from one formed during subsidence…. Lastly, I cannot understand Mr. Murray, who admits that small calcareous organisms are dissolved by the carbonic acid in the water at great depths, and that coral reefs, etc., etc., are likewise dissolved near the surface, but that this does not occur at intermediate depths, where he believes that the minute oceanic calcareous organisms accumulate until the bank reaches within the reef-building depth. But I suppose that I must have misunderstood him. Pray forgive me for troubling you at such length, but it has occurred to me that you might be disposed to give, after your wide experience, your judgment. If I am wrong, the sooner I am knocked on the head and annihilated, so much the better. It still seems to me a marvelous thing that there should not have been much and long-continued subsidence in the beds of the great oceans."

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