The ideas of Darwin and its contemporaries on the origin of the individual differences confused or were missed. The lamarckista concept predominated of that the environment makes to appear in the new individuals adaptativos characters, that if become hereditary. One of the first ones to approach experimentally the question was the German biologist August Weismann, still in century XIX. Having cut, for some generations, the tails of mice that used as reproductive, showed that nor therefore the descendants started to be born with lesser tails. Weismann also established the basic distinction between germinal cells and somatic cells. Origin of the races: The gnicas mutations, recombinations, the natural election, the migratory environment differences, movements and the isolation, geographic as in such a way reproductive, concur to modify the frequency of the genes in the populations of animals and are, thus, the main factors of the evolution. Two races geographically isolated they evolve independently and if they diversify each time more, until the differences in the reproductive agencies, or the sexual instincts, or in the number of chromosomes, they are great the point to become impossible or, when possible, producing the crossing between them of barren offspring. With this, the two races are changedded into distinct species, that is, populations incapable to change genes.
From there for ahead, exactly that the barriers come to disappear and the species start to share territory the same, it will not have between them viable crossings. The two species will form, forever, biological units stanch, of different evolutivos destinations. If, however, the geographic isolation between two races is precarious and disappears after some time, the crossing between them tends to obliterate the racial differentiation and them if they establish in one same species, monotypical, however very changeable. She is what human being is happening with the species, whose races if they had differentiated while the natural barriers were very difficult to be successful and had almost arrived at the point to form distinct species; but the ways of transport, introduced for the civilization, had been perfected before mechanisms of reproductive isolation were established that became the irreversible process.