SPECIES IN THE MAKING
A series of important conceptions are implied in the word "species," as
used by naturalists. Some of these we have noted in the last chapter.
There is first, as a starting-point, the conception that a species
is a number or company of individuals, all closely and clearly alike
(though presenting some minor individual variations), and capable of
sharp separation by certain "characters" from other similar groups or
companies. Then follows the addition (2) that the species is constant
if the conditions of life are not changed, or but little changed, and
that year after year it reproduces itself without change. It has a
certain stability (but not permanent immutability) greater in some
species than in others. Next we find (3) that the species constitutes
a group of individuals which have descended by natural breeding from
common parents, not differing greatly from the present individuals.
They are, in fact, one "stock." Then (4) that the species is a group,
the individuals of which pair with one another in breeding, but do not
pair with the individuals of another species, and that this is due to
various peculiar and inherent chemical, physiological and (in higher
animals) psychological characteristics of the species.
We have now further to note that species have their special
geographical _centres of origin_ from which most spread only a small
distance, whilst others have a wonderful power of dispersal, and
have become cosmopolitan. Moreover, we find that some species are
numerically very abundant, others very rare; that rare and abundant
species have often invaded each other's territory, and exist side by
side.
Whilst we often find a number of species, fifty or more, so much alike
that we unite them in a single genus (as, for instance, in the case
of the cats, lions, tigers, leopards, which form the genus "Felis,"
and the hundred or more species of the hedge brambles or blackberries,
which form the genus "Rubus"), there are many species which to-day
have, as it were, lost all their relatives and stand alone, the
solitary species in a well-marked genus, or have perhaps only one
other living co-species. And sometimes (curiously enough) that one
co-species is an inhabitant of a region very remote from that inhabited
by the other. Thus the two living mammals called tapirs (genus Tapirus)
inhabit, the one the Malay region, and the other Central America. This
is explained by the fact that tapirs formerly existed all over the
land-surfaces of North Europe, North Asia, and North America, which
connect these widely-separate spots. We find the bones and teeth of
the extinct tapirs embedded in the Tertiary deposits of the connecting
regions.
Once we have gained the fundamental conceptions as to what is meant by
a "species," we are able intelligently to consider innumerable facts
of the most diverse kind as to their peculiar structure and colours,
their number, localities, their interaction and dependence on other
living things, their modifications for special modes of life, their
isolation or their ubiquity. We can discuss their genetic relations
to one another, and to extinct fossil species, which have all been to
a very large extent "accounted for" or "explained" by Mr. Darwin's
theory of the origin of species by the natural selection of favoured
races in the struggle for existence. But there is always more to be
made out–difficulties to be removed, new instances to be studied. The
classification of the genera of plants and animals, with their included
species into larger groups, helps us to state and to remember their
actual build and structure, and to survey, as it were, the living
world, from the animalcule to the man, or from the microbe to the
magnolia tree. Every one interested in natural history should carry
in his mind as complete a scheme of the classification of animals and
plants as possible.
The older naturalists held that species were suddenly "created" as
they exist, and have propagated their like ever since. Darwin has
taught us that the present "species" have developed by a slow process
of transformation from preceding species, and these from other
predecessors, and so on to the remotest geologic ages and the dawn
of life. The agents at work have been "variation"–that is to say,
the response to the never-ceasing variation of the surrounding world
or environment–and the survival in the struggle for existence of the
fittest varieties so produced.
There is nothing surprising or extraordinary in the existence of
variation. The conditions of life and growth are never absolutely
identical in two individuals, and the wonder is not that species vary,
but that they vary so little. The living substance of animals and
plants is an extremely complex chemical substance, ever decomposing and
ever being renewed. It is the most "labile" as it is by far the most
elaborately built-up chemical body which chemists have ever ventured to
imagine. It differs, chemically, not only in every species but in every
individual and is incessantly acted upon–influenced as we may say–by
the ever-changing physical and chemical conditions around it. At the
same time it has, subject to the permanence of essential conditions,
a definite stability and limitation to its change or variation in
response to variations of its environment. That part of the living
substance which in all but the lowest plants and animals is set aside
during growth to form the eggs and sperms by which they multiply or
"reproduce" themselves, is called the "germ-plasm," and is peculiarly
sensitive to variations in (that is a _change_ in) the environment of
the plant or animal.
New conditions of life (locality and climate)–unusual food or
reproductive activity–act often in a powerful way upon the germ-plasm
and cause it to vary–that is to say, they alter some of its qualities,
though not necessarily disturbing in any way the general living
substance of the organism so far as to produce any important change
perceptible to the human eye. In consequence, the young produced after
such disturbance of the germ-plasm are found to differ more from their
parents than in cases where no such disturbance has been set up by the
natural never-ceasing variation of the surrounding world. This fact
is well known to horticulturists and breeders, and is made use of by
them. When a gardener wishes to obtain variations of a plant from which
to select and establish a new breed, he deliberately sets to work to
disturb–to shake up, to act upon in a tentative, experimental way–the
germ-plasm of one or more parent plants by change of soil, climate,
food and often by cross-fertilizing them with another breed or variety.
In this way he to some extent "breaks" the constitutional stability of
the germ-plasm of the plant and obtains abundant "variations" in the
offspring. These are not precisely foreseen, and show themselves in all
parts of the new generation. But some of them are what the gardener
wants, and are "selected" by him for retention, rearing and breeding.
The response of the germ-plasm of organisms to the stimulus of new
environmental conditions has been compared to that of the well-known
pattern-producing toy–the kaleidoscope. The bits of glass, beads and
silk which you see in a kaleidoscope, forming by reflection in its
mirrors a beautiful and definite pattern, are changed by a simple
vibration caused by tapping the instrument into a very different
pattern, the coloured fragments being displaced and rearranged. The
apparent change or variation is very great though produced by slight
mechanical disturbance, and the new pattern is altogether without any
special significance–the fortuitous outcome of a small displacement
of the constituent coloured fragments. We can imagine that similarly
slight disturbances of the organic molecules of the germ-plasm may
produce considerable and important variations in it and the new growth
to which it gives rise: and, further, that these variations may prove
to be either (1) injurious, or (2) of life-saving value, or often
enough (3) of no consequence whatever although bulking largely in our
human eyes and thereby misleading our judgment of them.
There is no reason to doubt that the same sequence of events occurs
in nature apart from man's interference. Changes occur in the earth's
surface, or the organism is transported by currents of water or
air into new conditions. The germ-plasm is "disturbed," "shaken"
or "shocked" by those new conditions, and a variation, in several
structures and qualities of the offspring subsequently produced,
follows. Then also follows the selection of one of the new varieties by
survival of the fitter to the new conditions into which the organism
has been transported or have developed in the region where it was
previously established.
This process of germ-variation is obviously as necessary and constant
a feature of the living organism as is the variation in the contour
of land and sea and in the extent of the polar ice-cap–a necessary
feature of the physical conditions of the terrestrial globe. But it
is the fashion with a certain school of writers nowadays to declare
that "variation" in organisms is a "mystery" unsolved. Another very
common and almost universal error is to overlook the fact that
variation is constitutional and affects whole systems of organs and
their deeply related parts, and is _not_, as it is so frequently and
erroneously assumed to be, a mere local affair of patches and scraps
visible on this or that part of the surface of an animal or plant.
These superficial "marks," readily seen and noted by the collector,
are rarely of any life-saving importance: they are but the outward and
visible signs of deep-lying physiological or constitutional change or
variation. The varying organism has, like Hamlet, "that within which
passeth show" and the superficial variations (like his "inky cloak"
and other customary features of mourning) are but "the trappings and
the suits" of a deep-lying change. Variation is not an inexplicable
mystery, nor, on the other hand, are "varieties" sufficiently dealt
with and their nature appreciated when one or two surface peculiarities
are enumerated by which the collector can recognize them. A deeper
study of the varying organism is both possible and needed.
If the gradual formation of new species from ancestral species is a
true account of the matter, we must expect to find, at any rate here
and there, if not frequently, traces of the process–for instance,
gradations, or series of intermediate forms, connecting new,
well-established species with the ancestral form or with one another.
We do find such gradations–sometimes more, sometimes less, completely
persisting over a wide tract of country, or discoverable in the
fossiliferous deposits containing the remains of extinct animals.
For instance, when we look at the butterflies of a much larger region
than our little island–namely, at those of a great continent like
Africa or South America–we find that there are species which show
gradations. Thus at a series of points, A, B, C, D, separated by some
hundreds of miles from each other, we find a corresponding series of
butterflies which are apparently closely similar species of one genus,
differing by a few spots of colour, or darker and lighter tint, much
as our Large White, Garden White, and Green-veined White differ. But
when the butterflies are caught which occur at points intermediate
between A and B, B and C, C and D, we find intermediate varieties, and,
in fact, if we get a very large number from intermediate regions, we
can, in some instances, arrange them in line so that they constitute
a graduated series of forms, each being scarcely distinguishable from
the one before or the one behind it, yet differing clearly from one a
dozen places away. In such cases there is often evidence to show that
the variety found at A breeds with that found at B, that of B with that
of C, of C with D, so that they form an inter-breeding group, though
perhaps the varieties at D will not pair with those at A, or even
with those at B. Then sometimes we find in such a series, otherwise
complete, a gap. Let us suppose it is between the butterflies of B and
C. We find the series of gradations nearly complete, but some natural
condition–such as the encroachment of the sea, or the slow elevation
of a mountain range, or the climatic destruction of the necessary
food-plant–has "wiped out" a few forms somewhere between those of B
and C. They no longer exist. The series is no longer connected by
inter-breeding forms; those occurring from A to B and some distance
beyond are one "species" varying in the direction of the series C
to D, but abruptly broken off from the latter. The series C to D is
also a "species" with graduated varieties, but distinct; it is cut
off from the lot once in continuity with it by the destruction of
the intermediate forms inhabiting an intermediate area. Thus the one
species becomes two, and these may again break up, and, having become
thus disconnected and stabilized, they may spread over one another's
territory–fly side by side and yet remain distinct forms which do not
pair together–although originally they were varieties spreading from a
common centre, where the ancestral species lived and multiplied.
Other similar gradational series of an interesting character have
been noticed in the case of fresh-water fossil snail-shells. In the
layers of clay and marl exposed by digging a railway cutting or a
pit we may find that the successive layers represent a continuous
deposit of 100,000 years or more, and we find sometimes that a form of
snail-shell (not a species living to-day) occurs in the lowest stratum
very different from that occurring in the highest stratum–the lowest
being short and spherical, the highest elongated and of differing
texture. In the intermediate layers, each 6 or 12 ins. thick and
occupying perhaps altogether 30 ft. of vertical thickness, we find a
graduated series of snail-shells leading almost imperceptibly from
the oldest lowest form to the latest uppermost form. Such cases are
known. But it is an exceptional thing to find these graduated series
either spread over an area of the earth's surface, or following one
another in successive strata. When they came into existence they were
rapidly superseded and destroyed as a rule, and have left only one
or two widely-separated examples of the intermediate forms. This we
should naturally expect by analogy from what we know of the successive
traces of human manufactures in the deposits on the site of some of the
great cities of the ancient world which have been carefully excavated
layer by layer. But still we have the important fact that here and
there such gradational series have been found, and we are justified
in considering a few isolated intermediate forms (which often occur
connecting two greatly-differing species) as survivors of a former
complete graduated series of intermediate forms, which came into
existence by slow modification of an ancestral stock, and may, when
the stock was widely spread over a continental area, not merely have
succeeded one another in time, but actually coexisted in neighbouring
regions.
There are many remarkable facts bearing upon the origin of "species,"
the description of which fills volumes written by such men as Darwin,
Wallace, Poulton, and others, and become interesting to every one who
has gained a correct notion of what naturalists mean by a "species." I
will cite one in order to illustrate this. The bird which we call the
red grouse, or nowadays simply "grouse" (the old Scotch name for it
was "muir-fowl"), is one of twenty-four birds (among the 400 species
of birds which live in the British Islands), including several kinds
of titmouse, the goldfinch, bullfinch, song-thrush, stonechat, jay,
dipper, and others which are very closely similar to species of birds
living in Continental Europe, yet show some definite and constant
marks, such as small differences in the colour of a group of feathers,
enabling us to distinguish the British from the Continental forms. Are
these twenty-four British forms to be regarded as distinct species?
The red grouse is placed in a genus called "Lagopus," of which there
are several species in the northern hemisphere. In Scotland the red
grouse, which is distinguished as Lagopus Scoticus, is accompanied by
a rarer species of Lagopus, which lives in high, bare regions. This is
the bird called by the Celtic name "ptarmigan"; it differs in several
points from the red grouse, and acquires white plumage in the winter,
which the latter bird does not; it is called Lagopus mutus. Now in
Norway we find also two species of grouse or Lagopus, called "rypé"
(pronounced "reeper") by the Norwegians. One is the same bird in every
respect as the Scotch ptarmigan, and is known as "the mountain rypé."
The other is very close to our red grouse, and is called "the common
or bush rypé," and by English naturalists the "willow grouse," and
by ornithologists "Lagopus salicetus." It agrees in habits, voice,
eggs, and anatomical detail with our red grouse, but the back of the
cock-bird of the red grouse and the whole plumage of the hen-bird
have a darker colour. Moreover, the willow grouse, like the ptarmigan
or mountain rypé, turns white–acquires a white plumage–in the winter
which the red grouse does not. Are the red grouse and the willow grouse
to be regarded as distinct species? Our British red grouse lives on
heather-grown moors; the willow grouse prefers the shrubby growths of
berry-bearing plants interspersed with willows, whence its name. Their
food differs accordingly. Formerly the red grouse lived on the moors
of the South of England, and when in Pleistocene times England was a
part of the Continent of Europe the willow grouse and the red grouse
were one undivided species inhabiting all the north-west of Europe. It
is probable, though the experiment would be almost impossible to carry
out, that were the eggs of a number of willow grouse now brought to
Scotland and hatched on the moors, they would tend to keep apart from
the native red grouse, and not inter-breed with them, in which case we
should say that the Scotch form is a "species on the make," or, even,
a completed and distinct species. On the other hand, it is possible
that the two forms would freely pair with another, and that the colour
and winter coat of the one (probably that of the Scotch form if the
experiment were tried in Scotland) would predominate, and after some
generations no trace of the other strain would be observable.