This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection.
Favourable variations are ones that increase chances for survival and procreation.
Reading of the struggle for existence that Malthus predicted, Darwin wrote in his autobiography, "It at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed...
Darwin had described evolution through the natural selection of accumulated favourable variations that in time formed new species; to Haeckel, however, this was only a beginning, with consequences to be pursued further.
Darwin's autobiography records his eureka moment: "I happened to read for amusement Malthus on population, and being well prepared to appreciate the struggle for existence…it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed.
"As natural selection acts solely by accumulating slight, successive, favourable variations, it can produce no great or sudden modification; it can act only by very short and slow steps," he wrote in The Origin of Species.
Although Darwin understood natural selection on a general basis, defining it as the "preservation of favourable variations, and the rejection of injurious variations" (1859), he did not know how it actually operated because genetics was unknown at the time.
"Natural selection acts solely by accumulating slight successive favourable variations, it can produce no great or sudden modification; it can act only by very short steps".
As species always breed beyond available resources, favourable variations would make organisms better at surviving and passing the variations on to their offspring, while unfavourable variations would be lost.
Ironically, Darwin's major, scheduled "Magnum opus" with the tentative title Natural Selection never appeared in print, but the Extract published by the author in November 1859 in order to establish priority with respect to his theory of the "preservation of favourable variations and the rejection of injurious variations" became a best- and longseller [ 6].
