And it was not so much the doubt cast by it upon the separate creation of particular species that was the disturbing element in Darwin's hypothesis (few men now regarded the book of _Genesis_ as a manual of natural science, or faith in it, as such, as a matter of religious obligation); it was rather that the new doctrine of "natural selection" seemed to invalidate the "argument from design." Design or chance had been the alternatives offered by Paley, and chance only had to be mentioned to be rejected; but Darwin made it possible to escape from the dilemma. He showed how, if certain conditions were granted, the whole process of the manufacture of species would naturally and inevitably follow. Neither design nor chance was the explanation: there was another alternative, _the influence of environment_. Thus Paley's instances of elaborate "contrivance" were explained by Darwin as instances of adaptation. The environment under which these organs had developed had made them what they were; they could not, under the given circumstances, have been different. As a very lucid writer puts it: "Before Darwin's great discovery, those who denied the existence of a Contriver were hard put to it to explain the appearance of contrivance. Darwin, within certain limits and on certain suppositions, provided an explanation. He showed how the most complicated and purposeful organs, if only they were useful to the species, might gradually arise out of random variations, continuously weeded by an unthinking process of elimination."[41] DARWINISM EXPLOITED.--In fact, it became evident that popular materialism had been strongly reinforced by the new biology; and though Darwin himself was cautious in adding philosophic or religious corollaries to his own propositions, some of his more eager disciples did not hesitate to fill in his blanks, and to draw conclusions which the master was too conservative, too blind, or perhaps too scientific to sanction. The distinguished zoologist Haeckel (1834-1919) may be reckoned the most notable amongst these. He was one of the first German scientists to give his adherence to Darwin, who seems to have considered him too zealous a disciple. "Your boldness sometimes makes me tremble," he wrote (November 19, 1868). It is not every scientist who can perceive the limits of an hypothesis, or who insists so conscientiously as Darwin did, upon the necessity for its verification.