Julian Huxley, biologist and humanist who would become the first president of UNESCO, wrote in 1942 a book of knowledge dissemination graduate Evolution: the modern synthesis. In it he refers to the first decades of the 20th century as "the eclipse of Darwinism". Huxley was the grandson of a great friend and supporter of Darwin, and knew the story firsthand. The problem with the theory of natural selection was that in those days there was no satisfactory explanation of the mechanisms of heredity, and without this it was very difficult for natural selection to be convincing. In fact, Darwin accepted the theory in vogue at the time, according to which all the cells of the body give off particles that mix in the offspring with the particles of the other parent. But this theory did not agree with the results of many experiments, especially some carried out on plants. Hence Huxley spoke of eclipse: without a good explanation of how characters are transmitted from one generation to the next, the Darwinian theory was no more than a hypothesis.

But while Darwin was publishing The Origin of Species, an Augustinian monk named Gregor Mendel was conducting experiments by crossing peas in his convent garden. His findings led him to postulate a theory of heredity in which characters did not mix, but appeared and disappeared in successive generations following precise laws and in fixed proportions. Although historians believe that Darwin never knew Mendel's work, we know that he read The Origin of Species and also Darwin's next book, in whose copies many personal annotations can be seen in the margins. In one of them, precisely when Darwin exposes his "hypothesis provisional" of the particles, Mendel writes that this is "a gratuitous assumption". And he expressly marks a paragraph where Darwin says that he "cannot give a satisfactory explanation" for some of his observations. Mendel had already given a satisfactory explanation in a article published in 1866, when he proposed the existence of the factors we know today as "genes" and described the laws of heredity.

But this publication fell into oblivion and therefore the Darwinian theory remained without a convincing explanation. It was not until 1900, when Mendel's work was rediscovered, that several scientists applied the laws of Genetics to the mechanisms of natural selection. With this, the eclipse of which Huxley spoke would come to an end and the new synthesis between Darwin's ideas and Mendel's discoveries would crystallize. This was an enormous step, thanks to which one of the founders of this new synthesis would affirm in the middle of the century that "nothing in biology makes sense if it is not in the light of evolution".

Darwin would enjoy immense fame, both during his lifetime and after his death in 1882. Mendel died two years later in complete anonymity; his recognition as the father of Genetics would take twenty years to arrive. That is why I think it is only fair that today, the day we commemorate the birth of the great English naturalist, we should also remember the scientist who made it possible for the theory of evolution to take off and consolidate. To paraphrase Newton's famous sentence, if today we can advance in fields as diverse as the fight to save the planet or cure diseases, it is largely because we can see farther because we have stood on the shoulders of these two giants.