About the beginning of the last century Geoffroy St. Hilaire, protégé, and in some respects a disciple of Buffon, was interested as to how living species are related to the animals and plants that had preceded them. He was familiar with the kind of change that takes place in the embryo if it is put into new or changed surroundings, and from this knowledge he concluded that as the surface of the earth slowly changed--as the carbon dioxide contents in the air altered--as land appeared--and as marine animals left the water to inhabit it, they or their embryos responded to the new conditions and those that responded favorably gave rise to new creations. As the environment changed the fauna and flora changed--change for change. Here we have a picture of progressive evolution that carries with it an idea of mechanical necessity. If there is anything mystical or even improbable in St. Hilaire's argument it does not appear on the surface; for he did not assume that the response to the new environment was always a favorable one or, as we say, an adaptation. He expressly stated that _if_ the response was unfavorable the individual or the race died out. He assumed that _sometimes_ the change might be favorable, i.e., that certain species, entire groups, would respond in a direction favorable to their existence in a new environment and these would come to inherit the earth. In this sense he anticipated certain phases of the natural selection theory of Darwin, but only in part; for his picture is not one of strife within and without the species, but rather the escape of the species from the old into a new world. If then we recognize the intimate bond in chemical constitution of living things and of the world in which they develop, what is there improbable in St. Hilaire's hypothesis? Why, in a word is not more credit given to St. Hilaire in modern evolutionary thought? The reasons are to be found, I think, first, in that the evidence to which he appealed was meagre and inconclusive; and, second, in that much of his special evidence does not seem to us to be applicable. For example the monstrous forms that development often assumes in a strange environment, and with which every embryologist is only too familiar, rarely if ever furnish combinations, as he supposed, that are capable of living. On the contrary, they lead rather to the final catastrophe of the organism. And lastly, St. Hilaire's appeal to sudden and great transformations, such as a crocodile's egg hatching into a bird, has exposed his view to too easy ridicule. But when all is said, St. Hilaire's conception of evolution contains elements that form the background of our thinking to-day, for taken broadly, the interaction between the organism and its environment was a mechanistic conception of evolution even though the details of the theory were inadequate to establish his contention.