Gould, in addition, told him that twenty-five of his twenty-six terricular birds from the Galàpagos were new species, and that the finches he had collected in the Archipelago belonged to thirteen different species.
It was at that point that Darwin became an evolutionist. As Lamarck before him, he discovered that evolution was needed in order to understand the present, to explain the diversity and the adaptations that we see today in the world around us. And as in Lamarck’s case, the reality of evolution could be grasped even if one did not understand its mechanism. The problem of the mechanism, however, remained, and Darwin started thinking about it.

Natural selection

In his Autobiography (1876) Darwin wrote that the idea of natural selection came to him essentially from two sources: from his talks with animal breeders, and from the Essay on the Principle of Populations by the reverend Thomas Malthus (1798). Darwin adds that he had the idea in October 1838, but this has been contested by some scholars because it would not explain why he waited 21 years before publishing it (and would have waited even longer if it had not been for Alfred Wallace).
In reality, however, there is no paradox in that delay. Darwin postponed publication because the Origin of Species had to deal with a great many consequences of natural selection for the history of life, and he wanted to argue them at lenght and to illustrate them with as many experimental facts as possible. Even in the Origin, at any rate, Darwin states that natural selection is the inevitable conclusion of four “undisputable” facts, two from Malthus and two from the breeders.
From Malthus he obtained these two conclusions:
(1) all populations can grow at an exponential rate, and
(2) the limited resources of the environment allow only a restricted growth.
The automatic consequence of these two facts is that in any population only some can survive, and we have therefore the problem of understanding what is it that decide their survival. Chance? Destiny?