He thus explained the origin of the giraffe's long neck in part through 'the inherited effects of the increased use of parts'. In seasons of limited food supply, Darwin reasoned, giraffes would stretch their necks for the high leaves, supposedly resulting in longer necks being passed on to their offspring. This involves shuffling the genes and is the reason that children resemble their parents very closely but are not exactly like either one. The discovery of the principles of recombination was Gregor Mendel's great contribution to the science of genetics. Mendel showed that while traits might be hidden for a generation they were not usually lost, and when new traits appeared it was because their genetic factors had been there all along.
Recombination makes it possible for there to be limited variation within the created kinds.
But it is limited because virtually all of the variations are produced by a reshuffling of the genes that are already there. For example, from 1800, plant breeders sought to increase the sugar content of the sugar beet. And they were very successful. But there the improvement stopped, and further selection did not increase the sugar content. Because all of the genes for sugar production had been gathered into a single variety and no further increase was possible.