If the reader will turn to the short notice of Aristoteles, he will find the ordinary idea of what a species means given. But Lamarck added something: “A species consists of a collection of individuals resembling each other, and reproducing their like by generation, so long as the surrounding conditions do not alter to such an extent as to cause their habits, characters, and forms to vary.” He stated what is only known to those naturalists who have had experience; the more we advance in the knowledge of the different organized bodies which cover the surface of the globe, the more our embarrassment increases to determine what ought to be regarded as a species, and still more how to limit and distinguish genera. In proportion as our collections are enriched, we see almost every void filled up, and all our lines of separation effaced; we are reduced to arbitrary determinations, and are sometimes fain to seize upon the slight differences of mere varieties in order to form characters for what we choose to call a species, and sometimes we are induced to pronounce individuals, but slightly differing, and which others regard as true species, to be varieties. The greater the abundance of natural objects assembled together, the more do we discover proofs that everything passes by invariable shades into something else; that even the more remarkable differences are evanescent, and that nature has, for the most part, left us nothing at our disposal for establishing distinctions save trifling, and in some instances puerile, peculiarities. We find that many genera amongst animals and plants are of such an extent, in consequence of the number of species referred to them, that the study and determination of these last has become almost impracticable. From a great number of facts we learn, wrote Lamarck, that in proportion as the individuals of one of our known species change their situation, climate, and manner of living, they change also little by little, the consistence and proportions of their parts, their form, their faculties, and even their organization, in such a manner that everything in them comes at last to participate in the mutations to which they have been exposed. Even in the same climate, a great difference of situation and exposure causes individuals to vary; but if these individuals continue to live, and to be reproduced under the same difference of circumstances, distinctions are brought about in them which become in some degree essential to their existence; and, in a word, at the end of many successive generations, these individuals which originally belonged to another species are transformed into a new and distinct species.