It's closer to a Darwin quote than a Hitler one. In Darwin's Descent of Man he explains the role of warfare in human evolution. Schools embrace Darwin's ideas on natural selection but ignore Darwin's book on human evolution, especially the key role tribal warfare plays in it.
Humans are the most genetically complex animal ever. Obviously something much faster than natural selection was going on. That something is our war making disposition. It's very important to understand the true nature of man, as leftist beliefs may have you exiting the gene pool.
In the animal world there are many variations on a theme, such as birds and fish. In humanoids there is only one species still alive. That's because we killed off in war all similar competitors long ago, the Neanderthals being the last. There are very little variations of a theme in modern humans, we are all remarkably the same. War is the reason behind this.
How do you measure genetic complexity?
What is the basis for this statement? What's your measure of genetic complexity? Total quantity of DNA? Number of genes? Number of chromosomes? Humans are unexceptional, genetically, by all the criteria that I can think of.
I can only guess you're assuming, rather than actually reading, what Darwin says about war in Descent. I've just searched through the full text, and he doesn't give it anything like the central role you suggest.
Darwin includes war, and pestilence, as amongst factors mitigating against "ratio of increase" (rapid population growth) but in the same passage and others surrounding suggests that the inability to obtain means of subsistence (principally good food) is far more important.
Darwin mentions war several times in discussing man's moral sense and his identity as a social animal. For instance:
When two tribes of primeval man, living in the same country, came into competition, if (other circumstances being equal) the one tribe included a great number of courageous, sympathetic and faithful members, who were always ready to warn each other of danger, to aid and defend each other, this tribe would succeed better and conquer the other. Let it be borne in mind how all- important in the never-ceasing wars of savages, fidelity and courage must be. The advantage which disciplined soldiers have over undisciplined hordes follows chiefly from the confidence which each man feels in his comrades. Obedience, as Mr. Bagehot has well shewn (5. See a remarkable series of articles on 'Physics and Politics,' in the 'Fortnightly Review,' Nov. 1867; April 1, 1868; July 1, 1869, since separately published.), is of the highest value, for any form of government is better than none. Selfish and contentious people will not cohere, and without coherence nothing can be effected. A tribe rich in the above qualities would spread and be victorious over other tribes: but in the course of time it would, judging from all past history, be in its turn overcome by some other tribe still more highly endowed. Thus the social and moral qualities would tend slowly to advance and be diffused throughout the world.
IOW the one specific role Darwin moots for warfare is to INCREASE and improve human morality, sociality, cooperativeness and selflessness. (True enough, that's exactly the opposite from how leftists would tend to see the matter.)
I can find no place, however, anywhere in the entire text of Descent, where Darwin says anything at all like the thesis you attribute to him: That warfare makes men less variable.
Indeed Darwin says repeatedly that mankind is a HIGHLY VARIABLE species.
That's because we killed off in war all similar competitors long ago, the Neanderthals being the last.
Speculation at best. There is scant evidence, and nothing conclusive, that anatomically modern humans were ever in direct competition (let alone at war with) Neanderthals. It is, under the present evidence, at least a viable hypothesis that such competition had NOTHING to do with the demise of Neanderthals.