"Or how can one study, or how can one be able to determine in a thorough way the species, among the multitude of known polyps of all orders of radiates, worms, and especially of insects, where the simple genera of Papilio, Phalæna, Noctua, Tinea, Musca, Ichneumon, Curculio, Capricorn, Scarabæus, Cetonia, etc., etc., already contain so many closely allied species which shade into each other, are almost confounded one with another? What a host of molluscan shells exist in every country and in all seas which elude our means of distinction, and exhaust our resources in this respect! Ascend to the fishes, to the reptiles, to the birds, even to the mammals, and you will see, except the lacunæ which are still to be filled, everywhere shadings which take place between allied species, even the genera, and where after the most industrious study we fail to establish good distinctions. Does not botany, which considers the other series, comprising the plants, offer us, in its different parts, a state of things perfectly similar? In short, what difficulties do not arise in the study and in the determination of species in the genera Lichena, Fucus, Carex, Poa, Piper, Euphorbia, Erica, Hieracium, Solanum, Geranium, Mimosa, etc., etc.? "When these genera were established but a small number of species were known, and then it was easy to distinguish them; but at present almost all the gaps between them are filled, and our specific differences are necessarily minute and very often insufficient. "From this state of things well established we see what are the causes which have given rise to them; we see whether nature possesses the means for this, and if observation has been able to give us our explanation of it. "A great many facts teach us that gradually as the individuals of one of our species change their situation, climate, mode of life, or habits, they thus receive influences which gradually change the consistence and the proportions of their parts, their form, their faculties, even their organization; so that all of them participate eventually in the changes which they have undergone. "In the same climate, very different situations and exposures at first cause simple variations in the individuals which are found exposed there; but, as time goes on, the continual differences of situation of individuals of which I have spoken, which live and successively reproduce in the same circumstances, give rise among them to differences which are, in some degree, essential to their being, in such a way that at the end of many successive generations these individuals, which originally belonged to another species, are at the end transformed into a new species, distinct from the other.