OF SPECIES AMONG LIVING BODIES AND THE IDEA THAT WE SHOULD ATTACH TO THAT WORD
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
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.
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
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.
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.
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.