During a voyage which he made in South America, Darwin had been struck by the very close affinity which existed between the living and the fossil species of this continent; this link between the past and present appeared to him to throw considerable light on the obscurity which enveloped the question of the origin of species. The degree in which organs were capable of modification was especially to be taken into account; the study of the variation of animals and plants under domestication led Darwin to the path he followed; the uninterrupted reproduction of characteristics in the structure of organic beings, intensified rather than attenuated by a succession of modifications, caused him to see in all living creatures, not independent entities, the one apart from the other, but descendants from common ancestors now extinct. Evolution, like many another theory, may be dangerous if not thoroughly grasped, and if it lead to a denial of the permanence of the well marked lines of demarcation in nature. Evolution, according to Darwin, starts from beginnings which are quite distinct; and leads on to well defined ends; thus Darwin does not acknowledge only one common progenitor for all the great natural races, but many, and nothing more clearly demonstrates his transparent sincerity in scientific matters than what his critics are pleased to call his inconsistencies. At the end of many years of persistent labour, Darwin published his book on the _Origin of Species_. I do not propose to give a summary of it. The author does not adopt the method of a learned writer expounding his system; his attitude is that of a naturalist who, during his excursions, examines nature in its innumerable and most minute details; when two facts, both of which he considers true, appear to contradict each other, he notes both equally, since he is too sincere to conceal that one from the public which apparently invalidates his theory. Moreover at each step he avows that this theory is not yet entirely free from the fog which invariably envelops each new idea at its birth. An explorer such as he is, who has succeeded in explaining so many mysteries, might very naturally become elated, but it is not so with him; his thoughts never seem directed towards himself; with all his genius, self does not appear to exist with him; the only things that are prominent--with a distinct existence--are the phenomena which he studies.