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"In Südamerika traten mir besonders drei Klassen von Erscheinungen sehr lebhaft vor die Seele: Erstens die Art und Weise, in welcher nahe verwandte Species einander vertreten und ersetzen, wenn man von Norden nach Süden geht; — Zweitens die nahe Verwandtschaft derjenigen Species, welche die Südamerika nahe gelegenen Inseln bewohnen, und derjenigen Species, welche diesem Festland eigenthümlich sind; dies setzte mich in tiefes Erstaunen, besonders die Verschiedenheit derjenigen Species, welche die nahe gelegenen Inseln des Galopagosarchipels bewohnen; — Drittens die nahe Beziehung der lebenden zahnlosen Säugethiere (Edentata) und Nagethiere (Rodentia) zu den ausgestorbenen Arten. Ich werde niemals mein Erstaunen vergessen, als ich ein riesengroßes Panzerstück ausgrub, ähnlich demjenigen eines lebenden Gürtelthiers.

"Als ich über diese Thatsachen nachdachte und einige ähnliche Er-

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scheinungen damit verglich, schien es mir wahrscheinlich, daß nahe verwandte Species von einer gemeinsamen Stammform abstammen könnten. Aber einige Jahre lang konnte ich nicht begreifen, wie eine jede Form so ausgezeichnet ihren besonderen Lebensverhältnissen angepaßt werden konnte. Ich begann darauf systematisch die Hausthiere und die Gartenpflanzen zu studiren, und sah nach einiger Zeit deutlich ein, daß die wichtigste umbildende Kraft in des Menschen Zuchtwahlvermögen liege, in feiner Benutzung auserlesener Individuen zur Nachzucht. Dadurch daß ich vielfach die Lebensweise und Sitten der Thiere studirt hatte, war ich darauf vorbereitet, den Kampf um's Dasein richtig zu würdigen; und meine geologischen Arbeiten gaben mir eine Vorstellung von der ungeheuren Länge der verflossenen Zeiträume. Als ich dann durch einen glücklichen Zufall das Buch von Malthus "über die Bevölkerung" las, tauchte der Gedanke der natürlichen Züchtung in mir auf. Unter allen den untergeordneten Punkten war der letzte, den ich schätzen lernte, die Bedeutung und Ursache des Divergenzprinzips".

 

[The English translation by E. Ray Lankester: Haeckel, The history of creation, 1876, vol. 1, pp. 134-5:]

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Letter from Charles Darwin to Haeckel, 8th October, 1864.

"In South America three classes of facts were brought strongly before my mind. Firstly, the manner in which closely allied species replace species in going southward. Secondly, the close affinity of the species inhabiting the islands near South America to those proper to the continent. This struck me profoundly, especially the difference of the species in the adjoining islets in the Galapagos Archipelago. Thirdly, the relation of the living Edentata and Rodentia to the extinct species. I shall never forget my astonishment when I dug out a gigantic piece of armour like that of the living armadillo.
"Reflecting on these facts, and collecting analogous ones, it seemed to me probable that allied species were descended from a common parent. But for some years I could not conceive how each form became so excellently adapted to its habits of life. I then began systematically to study domestic productions, and after a time saw clearly that man's selective power was the most important agent. I was. prepared, from having studied the habits of animals, to appreciate the struggle for existence, and my work in geology gave me some idea of the lapse of past time. Therefore, when I happened to read "Malthus on Population," the idea

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of natural selection flashed on me. Of all the minor points, the last which I appreciated was the importance and cause of the principle of divergence."

Darwin was asked to re-write his letter for Oscar Schmidt; it was published in O. Schmidt, The doctrine of descent and Darwinism. London: Henry S. King & Co. 2d ed. 1875, pp. 132-3:

"Having reflected much on the foregoing facts, it seemed to me probable that allied species were descended from a common ancestor. But during several years I could not conceive how each form could have been modified so as to become admirably adapted to its place in nature. I began, therefore, to study domesticated animals and cultivated plants, and after a time perceived that man's power of selecting and breeding from certain individuals was the most powerful of all means in the production of new races. Having attended to the habits of animals and their relations to the surrounding conditions, I was able to realize the severe struggle for existence to which all organisms are subjected; and my geological observations had allowed me to appreciate to a certain extent the duration of past geological periods. With my mind thus prepared I fortunately happened to read Malthus's "Essay on Population;" and the idea of natural selection through the struggle for existence at once occurred to me. Of all the subordinate points in the theory, the last which I understood was the cause of the tendency in the descendants from a common progenitor to diverge in character." ]