WHAT IS MEANT BY "A SPECIES"?
Those who take an interest in natural history must find it necessary
to know what the naturalist means by "a species" of animal or plant.
What does he mean when he says: "This is not the same species as
that," or "This is a species closely allied to this other species,"
or "This is a new species"? What are the "species" concerning the
origin of which Darwin propounded his great theory? There is really no
English word which can be exactly used in place of the word "species."
I often have to use the word when writing about plants or animals,
and should like once for all to say what is meant by it. One might
suppose that a "kind" is the same thing as a species. And so it often
is; but, on the other hand, by the word "kind" we often mean a group
including several species. For instance, we say the "cat-kind" or
the "daisy-kind," meaning the "cat-like" animals or the "daisy-like"
plants. The expression "the cat-kind" includes the common cat and the
wild cat, and even leopards, lions, and tigers, each of which is a
species of cat. And by the "daisy-kind" we understand a group including
several species of daisies, such as the common daisy, the ox-eye daisy,
the camomile daisy, the michaelmas daisy, and others. Hence we cannot
translate species simply by the word "kind." "Kind" is the same word
as "kin"–"a little more than kin and less than kind," runs Hamlet's
bitter pun. "Kind" means a group held together by kinship, and it may
be a larger or a smaller group held together by a close kinship or
by a more distant one. "Sort," again, will not serve our purpose as
an English translation of "species." For, although "a sort" implies
a certain selection and similarity of the things included in the
"sort," the amount of similarity implied may be very great or it may be
indefinitely vague and remote. Hence naturalists have to stick to the
word "species," and to use it with a clear definition of what they mean
by it.
Suppose we get together a large unsorted collection–many hundred
"specimens" or individuals–of the common butterflies of England. Then,
if we look them over, we shall find that we can pick out and arrange
the specimens into definite groups, according to their colour-pattern.
We find that the kinds which we readily distinguish are called in
English the swallow-tails, the whites, the sulphurs, the clouded
yellows, the tortoise-shells, the peacocks, the red admirals, the
painted ladies, the gatekeepers, the meadow browns, the heaths, the
coppers, and the blues. There might be others in such a collection, but
that is enough for our purpose. On examining the specimens closely, we
find that the colour-markings and "venation" or network by which the
wings are marked and the shape of the wings, body, and legs of all the
specimens of the swallow-tails are almost exactly alike, and unlike
those of any of the others. We shall find if we have a dozen or two
specimens that there is a slight difference in the pattern, size, and
colour of wing of some of the swallow-tails, dividing them into two
groups, which we soon ascertain to be the males and females; but this
is so small a difference that we may ignore it. The swallow-tail is
obviously and at once distinguished from any of the other butterflies
in the collection by its colour-pattern and shape. So also with the
others, there will be many specimens in each case agreeing in colour
and pattern, and recognizable and distinguishable from the rest by
the colour-pattern and by the "venation" or "nervures" of the wings.
If we collect butterflies again in other years and in other parts of
the country, we find the same set of shapes and patterns exactly,
corresponding to what we have learnt to call swallow-tails, whites,
sulphurs, clouded yellows, tortoise-shells, etc. There are, we thus
learn, several distinct, unchanging kinds of butterfly, which are
common in this country, and appear every year. Similarly we may go into
a meadow in spring, and gather a number of flowers, and a naturalist
will roughly arrange our bouquet into "kinds"; there will be the
buttercups, the daisies, the clovers, the dead nettles, the poppies,
the roses, the orchids, etc.
If, now, we look more carefully at our collection of butterflies,
sorted out roughly into kinds or species, we shall find that the
"whites," although holding together by a close similarity in
having merely white wings edged and spotted with black, yet differ
amongst themselves, so that we distinguish a larger kind, the large
garden-white, and a smaller, commoner kind, the smaller garden-white,
and we distinguish also the green-veined white, and possibly the
rare Bath white, each of them differing a little in their spots as
well as their size. These different sorts of "whites" can, once our
attention is drawn to the matter, be readily distinguished from one
another, and constantly are found in our collections. We thus arrive
at the conclusion that, though the whites are much alike, and are a
kind distinct from the other kinds of butterflies, yet the "whites"
themselves can be divided into and arranged as several kinds distinct
from one another. In fact, we discover (and an illustrated book on
butterflies confirms us in the conclusion) that there are several
ultimate kinds of whites which cannot be further separated into
groups. These are what are called "species." The whites are therefore
not a single species, as are our British swallow-tails, but a group of
species, closely related to one another. We find the same thing to be
true with regard to the blues. Though they are much alike, agreeing in
a variety of details of spotting and colour, yet we can distinguish the
chalk-hill blue, the common blue, the azure-blue, the Adonis blue, and
others, as distinct "species" of blues. Then, again, when we carefully
examine our English specimens of tortoise-shells, we find that there
are two distinct "species"–the greater and the smaller–differing not
only in size, but in pattern; and when we compare with these the
painted lady and the peacock and the red admiral, we find that there
is a certain agreement of wing-pattern (venation and outline) and
details of shape among them all, although their tints and the shape of
the spots and bands of colour differ. These different species "hold
together" just as the whites do and just as the blues do. Naturalists
have met the need for expressing this similarity of a number of
distinct species to one another by introducing the term "genus" for
such a group. In fact we arrange several species into a "genus."
The "genus" is a "kind," but a more comprehensive "kind," than is a
species. The species is an assemblage of _individuals_ closely alike to
one another; the genus is a group of _species_ which are more like to
one another than any of them are to other species.
Naturalists give to every genus a name, and also a name to each species
in the genus. Since we naturalists want to know what butterflies or
other species of animals and plants are found in other countries, and
to be sure that we all (whatever our native language may be) mean the
same thing by a name, Latin names are given to the genera and the
species, and are necessarily used when one wishes to be sure that
one is understood. The greatest trouble is taken to make certain
that the name used is applied only to the original species and the
original genus to which it was applied, for only so can one be sure
that a writer in America or one in Italy or France means the same
thing by a name as we do here in England. This is rendered possible
and is actually brought about by the preparation of catalogues in
which the species are described and figured, especially with regard to
obvious points of detail which are constant, and are called "specific
characters." These are chosen for special description, not haphazard,
but with a view to their being recognized with certainty by those
who study other specimens. Another extremely important proceeding
in connection with this purpose of uniform naming, which involves
vast labour and expense, is the maintenance of great collections of
preserved animals and plants by the State in all civilized countries.
In these collections either the original specimens to which names were
given by recognized describers (called "type-specimens" or "the type")
are preserved, or else specimens which have been compared with those
original described specimens, and authoritatively ascertained to be the
same as the "type." The maintenance of accuracy and agreement in regard
to the names of all the "species" of plants and animals is a big task.
It is now carried out by international councils, in which the skilled
naturalists of the world are represented. Certain principles have been
agreed upon as to the method of determining the priority of one name
over others which have been employed for one and the same species
by naturalists of different countries and at different times, and a
general agreement as to what names are to be used has been arrived
at. It is a matter which has involved a great deal of uncertainty
and dispute, and still causes difficulty. By the exercise of good
sense, and in consequence of the existence of an urgent desire really
to understand one another, there is now every year an increasing
uniformity and agreement among naturalists about the exact name to be
applied to every species of living thing.
Returning to our collections of butterflies and meadow flowers, we
may take the names of some of the species and genera as an example
of the system of naming in use by scientific naturalists. The common
swallow-tail is assigned to the genus Papilio. Its "specific name" is
"Machaon," given to it by Linnæus, hence it is spoken of as Papilio
Machaon. It is found in various parts of Europe as well as in England.
But in Central Europe (often seen in Switzerland) there is also
another species of swallow-tail, which only occurs as a rare accident
in England. This is the pale swallow-tail, differing, not only by its
paler colour but by definite spots and markings of the wings, from the
English species. Its species name, or "specific name," is "Podalirius,"
and so it is known as Papilio Podalirius. Species of Papilio are found
all over the world; more than 500 are known. Our two commonest whites
belong to the genus Pieris–they are called respectively Pieris brassicæ
(the larger) and Pieris rapæ (the smaller). The green-veined white is
Pieris napi. Each of these three is called after the plant, cabbage,
rape, or turnip, on which its caterpillar feeds. The rare Bath white is
Pieris daplidice. Its caterpillar feeds on mignonette. There are dozens
of species in other parts of the world allied to our "whites," which
naturalists have carefully distinguished and characterized by their
marks.
Several of our most beautiful species of English butterflies which are
much alike have been enrolled in one genus–the genus Vanessa. This
genus includes the great tortoise-shell, called Vanessa polychloros;
the smaller tortoise-shell, Vanessa urticæ; the peacock, Vanessa Io;
the painted lady, Vanessa cardui; the red admiral, Vanessa Atalanta;
and the comma butterfly, Vanessa C-album. There are other European,
Asiatic, and American species of Vanessa.
In the same way we find with our meadow plants that what we at
first thought was a single kind, "_the_" buttercup really bears a
name applicable to a genus in which are several common species. The
genus is called Ranunculus, and there are several common English
species with yellow flowers, but distinguished from one another by
definite characters. They are Ranunculus acris, Ranunculus flammula,
Ranunculus bulbosus, Ranunculus arvensis, Ranunculus ficaria (the
lesser celandine). And then there is the white-flowered Ranunculus
aquatilis–a common pond plant. Clover, again, is by no means the name
for a single species. The clovers form the genus Trifolium, and in any
English meadow we may come across the white clover, Trifolium repens;
the red clover, Trifolium pratense; the hop clover, Trifolium agrarium:
the strawberry clover, Trifolium fragiferum; the haresfoot clover,
Trifolium arvense. So it is with the plants which at first sight we
distinguish merely as "daisies." There are several distinct genera of
daisies–Aster, Bellis, Chrysanthemum (ox-eye), Anthemis (camomile), and
others, with several distinct species in each genus.
Enough has been said to show the reader that the mere notion of "kinds"
does not carry the same meaning as "species," but that there are a
number of regularly occurring definite forms of both animals and plants
which can be arranged in groups consisting only of individuals which
are very nearly identical with one another. A group of living things of
this degree of likeness is called "a species," and receives a name. A
less degree of likeness holds together a number of species to form what
we call a genus, and the name of the genus is cited together with the
name of the species when we wish to speak of the species with clearness
and certainty. This system of double names we owe to the great Swedish
naturalist of the eighteenth century, Linnæus. He proposed also that
the relationships of living things to one another should be further
expressed by grouping like genera into "families," then like families
into "orders," and like orders into "classes." And since his day we go
further and group classes into "phyla" or great stems of the animal
pedigree. In this way a complete hierarchy or system of less and more
comprehensive groups has been established, and is the means by which
we indicate the natural groups of the family-trees of plants and of
animals, what, in fact, is called the "classification" of each of these
great series of living things. Linnæus compared his system of groups to
the subdivisions of two armies. Thus, the one army represents the whole
animal series, the other the whole vegetable series. An army is divided
into (1) "legions," these into (2) "divisions," "divisions" into (3)
"regiments," regiments into (4) battalions, and battalions consist of
(5) companies, consisting of individual soldiers. According to Linnæus,
we may compare the legions to classes, which are divided into orders,
comparable to divisions; these into families, comparable to regiments;
these into genera, comparable to battalions; and these into species,
comparable to companies, or ultimate groups of individual units or
soldiers.
Just as the legions, divisions, regiments, battalions and companies
of an army have each their own name or at any rate a distinctive
numeral assigned to them in order that they may be cited and
directed, so are names given to each class, order, family, genus and
species of the classification or enumeration of the kinds of animals
and plants. Here, for instance, are the names of the greater and
smaller groups in which our common "white" finds itself enrolled.
_Class_–Insects. _Order_–Lepidoptera. _Family_–Pieridæ. _Genus_–Pieris.
_Species_–brassicæ.