Unfortunately it lacked, and still lacks, the necessary basis of indispensable proof for its most fundamental assumption, to-wit, "the inheritance of acquired characters," that is, the inheritance by the immediate offspring of those structural and functional changes or "acquirements" which came to the parents during their life because of their special use or disuse of parts and their individual reactions to environmental conditions. The young giraffe had a longer neck than it otherwise would have had because its parents had stretched their necks by continual reaching up to the leaves on the highest branches. The young man-thing of Glacial Times had weaker and less developed scalp muscles because its parents had gradually given up any considerable use of these muscles for twitching their heavy shocks of hair to frighten away the flies.
Then came Darwin with his natural selection explanation, a very different explanation from Lamarck's, and one also very plausible and logical. Darwin did not altogether disbelieve in Lamarck's theory; but he believed much more in his own. Later came the Neo-Darwinians, and they went the whole way of rejecting Lamarck's explanation entirely, and accepting the natural selection explanation as the wholly sufficient cause and the only one needed to explain all evolution. The leader of the Neo-Darwinians was August Weismann of the University of Freiburg. He had as followers most of the German natural philosophers.
What is this "natural selection" that we all know so well by name, and so little, I am afraid, by content? For natural selection is much more widely known as a dominating scientific dogma, accepted popularly with little question as a sufficient explanation of evolution, than as something to be itself explained and viewed with a proper scientific doubt. As a matter of fact, it is high time that it should be generally known that not many naturalists of standing today accept natural selection as a sufficient explanation of the thoroughly accepted fact of evolution, or even as the most important among the numerous probable contributing factors of evolution. Indeed there are many reputable naturalists who repudiate natural selection altogether, as an actual contributing factor in species-forming and descent, and concede its influence as an evolutionary control, only in most general relations.