The diversity of life, the reality of adaptation and the geological changes require, together, a world which is incompatible with the fixity of species, and we are bound to conclude that in the past species must have changed. This is Lamarck’s real message: evolution is a reality because it is the only idea which explains the world that we see around us. The mechanism is important to understand how evolution happened, but in order to prove that it did happen Lamarck’s argument is enough. Darwin himself, as we will see, came to this conclusion.

Darwin’s bet

Charles Darwin was born in the same year (1809) that Lamarck published, in Philosophie Zoologique, the first complete theory of evolution. Darwin became familiar with the transformation-of-species idea since his youth, because his grandfather Erasmus had written a poem on it, and his father Robert, beside a successful doctor, was a declared misbeliever, but their arguments did not convice him. After an attempt at studying medicine (demanded by his father) the young Darwin decided to follow his desire to became a naturalist, and at 19 he entered the theology faculty at Cambridge. In those times, the great majority of naturalists were Church ministers, and the professors that Darwin met in Cambridge (in particular John Henslow and Adam Sedwick) reinforced his belief that the religious explanation of nature – or natural theology – was a world view far more rational and scientific than the speculations of the transformists. Shortly after the end of his studies (in 1831) Darwin boarded HMS Beagle for a five years voyage around the world, and there is no doubt that at the beginnig of the voyage he was a firm believer, as he wrote in his Autobiography, in “the strict and literal truth of every word in the Bible”.
At the end of the voyage, instead, Darwin was full of doubts, and a year later, in 1837, became an evolutionist, even if he had not yet discovered the mechanism of natural selection, an idea that came to him after another year of meditations, in 1838.