In his own mind Darwin regarded that immortal work merely in the light of an abstract of his projected volumes. So immense were his collections and so voluminous his notes that the 'Origin of Species' itself seemed to him like a mere small portion of the contemplated publication. And indeed he did ultimately work out several other portions of his original plan in his detailed treatises on the Variation of Animals and Plants under Domestication, on the Effects of Cross and Self-Fertilisation, and on the Descent of Man and Sexual Selection. But the immense and unexpected vogue of his first volume, the almost immediate revolution which it caused in biological and general opinion, and the all but universal adhesion to his views of all the greatest and most rising naturalists, to a great extent saved him the trouble of carrying out in full the task he had originally contemplated as necessary. Younger and less occupied labourers took part of the work off their leader's hands; the great chief was left to prosecute his special researches in some special lines, and was relieved from the necessity of further proving in minuter detail what he had already proved with sufficient cogency to convince all but the wilfully blind or the hopelessly stupid. The extraordinary and unprecedented success of the 'Origin of Species' is the truest test of the advance it made upon all previous evolutionary theorising. Those who had never been convinced before were now convinced by sheer force of reasoning; those who believed and those who wavered had their faith confirmed into something like the reposeful calm of absolute certitude. Let us consider, therefore, what exactly were the additions which Charles Darwin offered in his epoch-making work to the pre-existing conceptions of evolutionists. In 1852, seven years before the publication of Darwin's masterpiece, Mr. Herbert Spencer wrote as follows in an essay in the 'Leader' on creation and evolution. The expressions of so profound and philosophical a biologist may be regarded as the high-water mark of evolutionary thinking up to the date of the appearance of Wallace and Darwin's theory:--