Darwin essentially argued that living things are engaged in a 'struggle for existence' and that any features they possess which affect their survival and reproductive potential will be subjected to continuous selective pressure. This might be seen as his 'new big idea' in the whole evolution thing. This means that through time those features which confer an advantage in the struggle for life will be retained and those that constitute a handicap will be lost. Not for individuals but for the race. So the composition of the population (or species) changes through time, eventually producing such widely divergent forms that cannot interbreed, and are so called separate species. That is a brief outline of the theory.
While on HMS Beagle, he made detailed observations in the Galapagos that finches on different islands seemed to be derived from the same 'basic finch' through adapting to different diets and evolving different shaped bills. Thus you can see finches with seed-eating bills and those adapted for probing or insects or fruit eating. The bills were distinctly different, but not the rest of their bodies. These initial observations challenged everything the young Darwin had been taught at Cambridge and echoed his own grandfather's conjectures about the mutability of a species - very much an heterodox opinion at that time.
Thus, evidence-driven, Darwin was more or less forced to conclude that these different finches were essentially different races of the same basic finch which had spread out and then become geographically separated into different groups. Thus the plasticity of the species is what he really started out with. Having proved that to his own satisfaction, he then proceeded to work out the mechanism --including the struggle for life concept, the survival of the best fit and the weeding out of the unfit. The word fit being in some ways an ambiguous and unfortunate choice of words, and in fact meaning 'suited' or 'adapted' and NOT fit as in 'sporty, fast or trim'.
So the biological significance of race is immense --when you are looking at the animal world. But as soon as you transfer all this to the human world, various problems show up. Darwin himself was initially very reluctant to get into that side of it and left much of this work to his 'bulldog' Thomas Huxley, who presented evolutionism to the masses. He also left till fairly late in his life the whole highly vexed question of human evolution. That was probably because he knew it was a highly contentious field, especially in uptight, Anglican, Victorian England! He was afraid of upsetting people and deeply disturbed by thought of the religious…