p. 35
Chapter III

    It is not a futile purpose to decide definitely what we mean by the so-called species among living  bodies, and to enquire if it is true that species are of absolute constancy, as old as nature, and have all existed from the beginning just as we see them to-day; or if, as a result of changes in their environment, albeit extremely slow, they have not in course of time changed their characters and shape.


    Any collection of like individuals which were produced by others similar to themselves is called a species.

    This definition is exact: for every individual possessing life always resembles very closely those from which it sprang; but to this definition is added the allegation that the individuals composing a species never vary in their specific characters, and consequently that species have an absolute constancy in nature.

    It is just this allegation that I propose to attack, since clear proofs drawn from observation show that it is ill-founded.

p. 38
    How great the difficulty now is of studying and satisfactorily deciding on species among that multitude of every kind of polyps, radiarians, worms, and especially insects, such as butterflies, Phalaena, Noctua, Tinea, flies, Ichneumon, Curculio, Cerambix, chafers, rose-chafers, etc.! These genera alone possess so many species which merge indefinably into one another.

    What a swarm of mollusc shells are furnished by every country and every sea, eluding our means of distinction and draining our resources.

    Consider again, fishes, reptiles, birds and even mammals; you will see that except for gaps still to be filled, neighbouring species and even genera are separated by the finest differences, so that we have scarcely any foothold for setting up sound distinctions.

    Is there not an exactly similar state of affairs in the case of botany, which deals with the other series, consisting of plants?

    How great indeed are the difficulties of the study and determination of species in the genera Lichen, Fucus, Carex, Poa, Piper, Euphorbia, Erica, Hieracium, Solanum, Geranium, Mimosa, etc., etc.

    When these genera were constituted only a small number of species belonging to them were known, and it was then easy to distinguish them; but now that nearly all the gaps are filled, our specific differences are necessarily minute and usually inadequate.


p. 39
    The idea of bringing together under the name of species a collection of like individuals, which perpetuate themselves unchanged by reproduction and are as old as nature, involved the assumption that the individuals of one species could not unite in reproductive acts with individuals of another species.

    Unfortunately, observation has proved and continues every day to prove that this assumption is unwarranted; for the hybrids so common among plants, and the copulations so often noticed between animals of very different species, disclose the fact that the boundaries between these alleged constant species are not so impassable as had been imagined.

    It is true that often nothing results from these strange copulations, especially when the animals are very disparate; and when anything does happen the resulting individuals are usually infertile; but we also know that when there is less disparity these defects do not occur.  Now this cause is by itself sufficient gradually to create varieties, which then become races, and in the course of time constitute what we call species.


p. 44
    Thus, among living bodies, nature, as I have already said, definitely contains nothing but individuals which succeed one another by reproduction and spring from one another; but the species among them have only a relative constancy and are only invariable temporarily.

    Nevertheless, to facilitate the study and knowledge of so many different bodies it is useful to give the name of species to any collection of like individuals perpetuated by reproduction without change, so long as their environment does not alter enough to cause variations in their habits, character and shape.