"In the first lectur I laid stress on the great importance of Natural Selection – the selective deathrate – as tending to human efficiency,"
I will note that the goal of Pearson and all eugenicists is entirely in line with the theory of natural selection, to eliminate descendants of those selected through assignment of inferiority to them. In the case of Darwin and Galton who thought birth control was icky, they favored it happening through a violent struggle for existence. "Moderate" Darwinists, eugenicists, favored doing so, not through, necessarily, outright murder but through either forced or coerced sterilization of those, putting off their elimination for one generation so as to avoid getting called murderers. If the Nazis had sterilized all of the Jews instead of gassing them they would have not had a different goal or different means than the eugenicists advocated and, in fact, still advocate. They took a more purely Darwinian course of violence. That's the only difference.
Eugenics was a British Fabian article of faith and at many a Fabian gathering, even before the founding of the Nazi party that great wit and man of letters, George Bernard Shaw was among the several Fabians and other progressive intellectuals and scientists advocating the immediate gassing of those he deemed unfit. You can find the same things being said in the United States and elsewhere. Reading that great hero of the secular, non-Marxist left, Sidney Webb's comments about the Irish and Jews in that same post - his pamphlet where he said it also linked to - and the Chinese I wonder if anyone pointed out to Shaw that the Irish could be put on a hit list as easily as any other group desired. The Nazis chose one of Webb's chosen menaces for theirs. Though eventually eugenic thinking will always be able to identify ever newer groups of the inferior to eliminate to ever improve the human stock. If you get rid of "the lowest 10%" the next higher 10% moves into the position of "the lowest". Such is the amoral logic of such scientific valuation and grading of people and whatever set they choose to consider people as being part of.
If you want to see what such thinking can produce in real reality instead of Darwinian fantasy of ever greater selective fitness, look at the number of inbred physical problems and deformities selectively bred into show dogs, horses, etc. and that's even with the cruel and repulsive practice of killing those deemed unfit by their commercial breeders. Look at the British royal family over the generations.
Hate Mail Update: Geesh, do you think they didn't mean it when they said things like that AS RELIABLE SCIENTIFIC FACT? When Darwin and Huxley and others talked, explicitly about the "struggle for existence" and said such things as:
At some future period, not very distant as measured by centuries, the civilised races of man will almost certainly exterminate, and replace, the savage races throughout the world. At the same time the anthropomorphous apes, as Professor Schaaffhausen has remarked (18. 'Anthropological Review,' April 1867, p. 236.), will no doubt be exterminated. The break between man and his nearest allies will then be wider, for it will intervene between man in a more civilised state, as we may hope, even than the Caucasian, and some ape as low as a baboon, instead of as now between the negro or Australian and the gorilla.