"For example, if the seeds of a grass, or of every other plant natural to a humid field, should be transplanted, by an accident, at first to the slope of a neighboring hill, where the soil, although more elevated, would yet be quite cool (_frais_) so as to allow the plant to live, and then after having lived there, and passed through many generations there, it should gradually reach the poor and almost arid soil of a mountain side--if the plant should thrive and live there and perpetuate itself during a series of generations, it would then be so changed that the botanists who should find it there would describe it as a separate species. "The same thing happens to animals which circumstances have forced to change their climate, manner of living, and habits; but for these the influences of the causes which I have just cited need still more time than in the case of plants to produce the notable changes in the individuals, though in the long run, however, they always succeed in bringing them about. "The idea of defining under the word _species_ a collection of similar individuals which perpetuate the same by generation, and which have existed thus as anciently as nature, implies the necessity that the individuals of one and the same species cannot mix, in their acts of generation, with the individuals of a different species. Unfortunately observation has proved, and still proves every day, that this consideration has no basis; for the hybrids, very common among plants, and the unions which are often observed between the individuals of very different species among animals, have made us perceive that the limits between these species, supposed to be constant, are not so rigid as is supposed. "In truth, nothing often results from these singular unions, especially when they are very incongruous, as the individuals which result from them are usually sterile; but also, when the disparities are less great, it is known that the drawbacks (_défauts_) with which it has to do no longer exist. However, this means alone suffices to gradually create the varieties which have afterwards arisen from races, and which, with time, constitute that which we call _species_. "To judge whether the idea which is formed of species has any real foundation, let us return to the considerations which I have already stated; they are, namely-- "1. That all the organic bodies of our globe are veritable productions of nature, which she has created in succession at the end of much time. "2. That in her course nature has begun, and begins anew every day, by forming the simplest organic bodies, and that she directly forms only these--that is to say, only these first primitive germs (_ébauches_) of organization, which have been badly characterized by the expression of "spontaneous generations" (_qu'on a désignées mal-à-propos par l'expression de Générations spontanées_).