SOME SPECIFIC CHARACTERS
An interesting case, showing that qualities which are life-preserving
under certain severe conditions exist in some varieties of a species
and not in others, was recorded some eight years ago. After a very
severe "blizzard" 136 common sparrows were found benumbed on the ground
by Professor Bumpus at Providence, United States. They were brought
into a warm room and laid on the floor. After a short time seventy-two
revived and sixty-four perished. They were compared to see if the
survivors were distinguished by any measurable character from those
which died. It was found that the survivors were smaller birds (the
sexes and young birds being separately compared) than those which died,
and were lighter in weight by one-twenty-fifth than the latter. Also,
the birds which survived had a decidedly longer breastbone than those
which died.
Similarly, the late Professor Weldon found that in the young of the
common shore-crab, taken in certain parts of Plymouth harbour, those
with a little peculiarity in the shape of the front of the shell
survived when those without this peculiarity died. Many thousands were
collected and measured in this experiment. It is not necessary to
suppose that the distinguishing mark of the survivors in such cases
is "the cause" of their survival. Such marks as the breadth of the
front part of the crab's shell and the length of a bird's breastbone
very probably are but "the outward and visible signs of an inward and
(physiological) grace."
The marks, little peculiarities of colour and proportionate size,
or some peculiar knob or horn, by which the student of species
distinguishes one constant form from another, can rarely, if ever, be
shown to have in themselves an active value in aiding or saving the
life of the species of plant or animal. The mark or "character" is an
accompaniment of a chemical, nutritional, physiological condition,
and is in itself of no account. It is what is called "a correlated
character." Such, for instance, is the black colour of the skin
of pigs which in Virginia, U.S., are found, as stated by Darwin,
not to be poisoned by a marsh plant ("the paint-root," Lachnanthes
tinctoria), whilst all other coloured and colourless pigs are. The pigs
which are not black develop a disease of their hoofs which rot and
fall off, causing their death when they eat this special plant "the
paint-root." The colour does not save the pig–it cannot correctly be
called the _cause_ of the pig's survival–but is an accompaniment of the
physiological quality which enables the pig to resist the poisonous
herb. So, too, with white-spotted animals. They are known to breeders
as being liable to diseases from which others are free. Fantail pigeons
have extra vertebræ in their tails, and pouter pigeons have their
vertebræ increased in number and size. But the vertebræ were never
thought of and "selected" by the breeders. They only wanted a fanlike
set of tail feathers in the one case, and a longer body in the other.
Some varieties of feathering maintained by pigeon breeders lead to the
growth of abundant feathers on the legs (as in Cochin-China fowls), and
it is found that these feather-legged pigeons always have the two outer
toes connected by a web of skin. If it were a stabilized wild form we
should separate it as a species on account of its webbed toes, yet the
real selection and survival in the hands of the breeder had nothing to
do with the toes or their web, but was simply "caused" by these pigeons
having feathers of "survival or selection value" in his judgment. Male
white cats with blue eyes are deaf. If deafness were ever an advantage
(a difficult thing to imagine), you would get a species of cat with
white hair and blue eyes, and be led to distinguish the species by
those characters, not by the real cause of survival, namely, deafness.
Not enough is yet known of this curious and very important subject
of correlation, but its bearing on the significance of "specific
characters" is sufficiently indicated by what I have said.
An interesting group of species, three of which are to be purchased
alive through London fishmongers, are the European crayfishes, not to
be confused with the rock-lobster or Langouste (Palinurus), sometimes
called "crawfish" in London, nor with the Dublin prawn (Nephrops).
The little river crayfishes are like small lobsters, and were placed
by older naturalists in one genus with the lobsters. Now we keep the
European species of crayfishes as the genus Astacus, and the common
lobster and the American lobster have been put (by H. Milne-Edwards)
into a separate genus (Homarus). You can buy in London the "écrevisses
à pattes rouges" of French and German rivers, which is called Astacus
fluviatilis, and differs from that of the Thames and other English and
European rivers (which you can also buy) called A. pallipes ("pattes
blanches" of the French), by the bright orange-red tips of its legs,
and by having the side teeth of the horn or beak at the front of the
head larger and more distinct. The English crayfish grows to be nearly
as large as the "pattes rouges" in the Avon at Salisbury, though it has
nearly disappeared about Oxford. You can also sometimes buy in London
the big, long-clawed Astacus leptodactylus of East Europe. There are
two or three other species, named and distinguished, which do not come
into the London market.
Crayfishes, lobsters and the like have groups of plume-like gills
(corresponding in the most ancient forms to the number of the legs
and jaw-legs) overhung and hidden by the sides of the great shield or
"head" of the animal. The common lobsters and crayfishes retain most
of these in full size and activity, but have lost in the course of
geologic ages the original complete number. These plume-like gills–each
half an inch or so in length–are attached, some to the bases of the
legs and some to the sides of the body above the legs. In the ancestral
form there were thirty-two plumes on each side, twenty-four attached
to the bases of the legs, and eight placed each at some distance above
the connection of one of the eight legs with the side of the body. It
is those on the side of the body which have suffered most diminution
in the course of the development of modern crayfishes (and lobsters)
from the ancestral form provided with the full equipment of thirty-two
gill-plumes on each side. In fact, only one _well-grown_ gill-plume,
out of the eight which should exist on each side of the body-wall,
is to be found–and that is the one placed above the insertion of the
hindermost or eighth of the eight legs (eight when we reckon the
three jaw-legs as "legs" as well as the five walking-legs). In front
of this the side or wall of the body is bare of gill-plumes though
they are present in full size on the basal part of most of the legs.
Nevertheless, when one examines carefully with a lens the bare side
of the body overhung by the head-shield or "carapace," one finds in a
specimen of the common English "pale-footed crayfish" a very minute
gill-plume high above the articulation of the seventh leg and another
above the articulation of the sixth leg. They are small dwindled
things, as though on the way to extinction, and are the mere vestiges
of what were once well-grown gill-plumes, and still are so in the rock
lobster and some prawns. In the red-footed crayfish of the Continent
(Astacus fluviatilis) yet another minute vestige of a gill-plume is
found, farther in front, on the body-wall above the fifth leg on
each side of the animal. This furnishes a definite mark or character
by which we can distinguish the red-footed crayfish from the common
English pale-footed one. But these three rudimentary gill-plumes in the
red-foot species, and two in the pale-foot species are all that until
lately were recorded. The region of the body-wall above the fourth,
third, second, and first of the legs was declared to be devoid even of
a vestige of the branchial plumes which were there in ancestral forms,
and have been retained more or less in some exceptional prawn-like
creatures allied to the crayfish.
Zoologists take a special interest in the crayfish because it is found
to be a most convenient type for the purpose of teaching the principles
of zoology to young students, and with that end in view was made the
subject of a very beautiful little book by the great teacher Huxley.
The conclusions above stated in regard to the gills are set forth in
that book with admirable illustrative drawings, and the striking fact
of the dwindling and suppression of the various gill-plumes is clearly
explained.
[Illustration: FIG. 33.–The rudimentary gill-plume of a crayfish
from that part of the body-wall to which the first pair of jaw-legs
(maxillipedes) is articulated. Found in the red-footed crayfish
(Astacus fluviatilis) but in no other species of Astacus. It is
one-fifteenth of an inch long. Drawn by Miss Margery Moseley in 1904.
("Quart. Journal of Microscopical Science," vol. 26 (1904-5).)]
And now we come to an interesting discovery in this matter of the
gill-plumes of crayfishes. Some fifteen years ago the daughter of my
friend and colleague–Professor Moseley–was a member of the class of
Elementary Biology at Oxford. She had to examine and identify these
and other points in the structure of the crayfish. The class was
supplied with specimens of the French red-footed crayfish "Astacus
fluviatilis," as it is more readily obtained from fishmongers than
our own "pale-foot" or "Astacus pallipes." She found in her specimen
far forward on each side of the "head" a very minute gill far away
from the others and previously unknown. The demonstrator in charge of
the class refused even to look at her discovery. So she confirmed it
by examining three other specimens–made drawings of the tiny branched
gill (as shown in Fig. 33) and their position, and sent them to me
in London. It was at once clear that she had discovered in this much
studied little animal a very interesting pair of gills (right and
left)–unknown to Huxley and the rest of the zoological world. She
proceeded to examine specimens of A. fluviatilis from various rivers of
Germany and France and always found the new gill-plume. She also showed
(I supplied her with specimens at the Natural History Museum) that it
was, on the other hand, absent from A. leptodactylus, A. pallipes,
and all the foreign species (some from Asia) which are known, and she
published an illustrated account of it in the "Quarterly Journal of
Microscopical Science." This tiny gill-plume is placed very far forward
on each side of the body, the farthest point forward at which any
gill-plume is found in any kind of prawn, shrimp or lobster, namely
in the region where the first pair of jaw-legs is attached, so that
there are three empty spaces between it and the rudimentary gill over
the fifth pair of legs, already known in the red-footed crayfish. It
is only two millimetres long–about one-fifteenth of an inch! But its
presence serves very distinctly to separate the red-footed crayfish,
Astacus fluviatilis of French and German rivers, thus discovered
to have four pairs of rudimentary gill-plumes, from the Astacus
leptodactylus of the Danube basin and East Europe, which has only three
pairs, and still more to emphasize the difference between it and our
British species, the "white-foot" or Astacus pallipes, which has only
two!
This little history is noteworthy, firstly, because it shows that a
young student may, to use an appropriate term, "wipe the eye" of an
expert observer and rightly venerated teacher (who would have delighted
in the little discovery had he been alive), as well as the eyes of
tens of thousands of students and teachers (including myself) who have
studied the red-foot crayfish year after year, and missed the little
gill. It is also interesting as showing us a good sample of a specific
mark or character which has no survival value; that is, could not
advantage the crayfish in the struggle for life. The fact is, that this
one particular very minute forward pair of gill-plumes is like the
other rudimentary gills–a survival in a reduced condition of a pair of
gill-plumes which were well-grown, useful plumes aerating the blood,
in the prawn-like ancestors of all crayfishes, lobsters, shrimps, and
prawns, and is, owing to circumstances of nutrition and growth which we
know nothing about but can vaguely imagine, retained by the red-foot
species of crayfish, but lost by all other crayfishes, lobsters,
common prawns and shrimps, and, in fact, only retained besides by a
very few out-of-the-way kinds of marine prawns. That is the sort of
thing which frequently has to serve as "a specific character" or mark,
distinguishing one "species" from another.
A more ample discussion of the origin of species is not within the
scope of this book. But I may say that until recently the conception
that _every_ organ, part and feature of a plant and animal _must_
be explained, and can _only_ be explained, as being of life-saving
value to its possessor, and accordingly "selected" and preserved in
the struggle for existence, was held by many "Darwinians" in too
uncompromising a spirit. This conception was, really from the first,
qualified by the admission that the life-saving value and consequent
preservation of a structure must undoubtedly in some cases have been
in operation in ancestors of the existing species, and is no longer
operative in their descendants although they inherit the structure
which has now become useless. Moreover, the operation of those subtle
laws of nutrition and of form which are spoken of as the "correlation
of parts in growth and in variation" (mentioned on p. 119) was pointed
out by Darwin himself as probably accounting for many remarkable
growths, structures and colour-marks which we cannot imagine to be
now, or ever to have been in past ancestry, of a life-saving value.
Nevertheless, the old "teleology," according to which, in pre-Darwinian
days, it was held that every part and feature of an animal or plant has
been specially created to fulfil a definite pre-ordained function or
useful purpose, still influenced the minds of many naturalists. Natural
selection and survival of the fittest were reconciled with the old
teleological scheme, and it was held that we must as good Darwinians
account for every structure and distinctive feature in every animal and
plant as due to its life-saving value. Herbert Spencer's term, "the
survival of the _fittest_," conduced to the diffusion of this extreme
view: Darwin's equivalent term, "the preservation of _favoured_ races,"
did not raise the question of greater or less fitness.
The extreme view is now, however, giving place to the recognition
of the fact that the actual tendencies to variation–accumulated in
the living substance of the various stocks or lines of descent and
handed on during an immense succession of ages of change by hereditary
transmission–counts for more in the production of new species and
strange, divergent, even grotesque forms of both animals and plants
than had been supposed.
Undoubtedly selection or survival of the fittest mainly accounts for
the colouring and adaptive shaping of living things, and so for those
several great types of modelling which arrest the eye and have excited
the interest of inquisitive man. But there seems to be no justification
for the assumption that in all cases a variation–that is to say, an
increase or a diminution of the volume of some existing structure in
proportion to other coexisting structures in the body of a living plant
or animal–must be _either_ favourable, that is, conducive to survival,
_or_ injurious, that is, tending to the defeat and destruction of its
possessors or their race. On the contrary, it is the fact that there
are vast areas and conditions related to countless myriads of living
creatures in which variations of those creatures of large and imposing
kind and degree are neither advantageous nor disadvantageous, but
matters of _absolute indifference_, that is to say, without any effect
upon the preservation or survival of their race or stock. Nature is
far more tolerant than some of us were inclined to assume. In certain
restricted conditions of competition and in regard to some special
structures and components which are often so minute and obscure as
to be not yet detected by that recent arrival, the investigating
biologist–though sometimes, fortunately for him, large enough to
jump to his eyes–it is undeniable that there must be a "survival" or
"favouring" of individuals presenting a variation in increase, or it
may be decreased, of this or that special feature of its "make-up" or
structural components. But it is a more correct statement of the case
to say that natural selection or survival preserves _not the fittest_,
but _the least fit possible_ under the circumstances–namely, all those
which, however great their divagations and eccentricities of variation
in other respects, yet at the same time attain to a minimum standard
of qualification in those structures (or inner chemical qualities)
essential for success in the competition for safety, food and mating
determined by the particular conditions in which the competition is
taking place. Consequently forms which are meaningless so far as
standards of utility or "life-saving" are concerned, and are rightly
described as grotesque, monstrous, gigantic or dwarfed–excessive (as
compared with more familiar kinds) in hypertrophy or atrophy of their
colouring and clothing, or of out-growths such as leaves of plants and
limbs, jaws or other regions of the body of animals–are found existing
in various degrees of eccentricity in every class of both plants and
animals. Among animals such tolerated "exuberances of non-significant
growth" are more striking than in plants. The group of fishes seems
to be especially privileged in this way. They are freely variable in
the position of the fins, the suppression or exaggeration of them, as
well as of the scales on the surface of the body (_e.g._ leather carp
and mirror carp). Take, for example, the mackerel and the salmon as
standards of utilitarian adaptation of the body to an active life in
sea or river, and then compare with theirs the astounding proportions
of the sun-fish (Orthagoriscus) like a cherub "all head and no body,"
or the almost incredible Pteraclis–with its little body framed
immovably between a huge dorsal and a huge ventral fin (see figures
on p. 130). The fin-like crest of enormous size on the back of the
great extinct lizard Dimetrodon of the Permian age supported by long
bony spines is a similarly excessive and useless outgrowth. (This
astonishing creature is shown in our Frontispiece.) Such exuberant
products may be ascribed to an unrestrained "momentum" of growth
which once set going by fortuitous variation has been _tolerated_
but not _favoured_ by natural selection. Or (as supposed by some)
their excessive development may be due to the _persistence_ of some
nutritional condition which at first resulted in a moderate growth of
the fin-like crests in question as a serviceable structure, but has
persisted and increased long after the fin or crest has attained a
sufficient size–simply because its increase though of no life-saving
value–yet was not harmful and so did not bring its owner under the
guillotine of natural selection. Such disproportionate exuberance of
growth due to innate variability, tolerated but not specially favoured
by natural selection, will account for many strange and grotesque forms
of living things. From time to time in the long process of change, such
exuberances may suddenly become of service and be, so to speak, taken
in hand by natural selection, or they may become dangerous and lead
to the extermination of the stock in which they have been previously
tolerated.
Before my reader turns–as I hope he or she will do–to some handbook of
zoology in which the genealogical tree or classification of the species
of animals and of plants is treated at length, I will endeavour to
give some estimate of the immense numbers of "species" which exist. As
to mere individuals, it is impossible to form any estimate, but when
we reckon up the teaming population of a meadow or forest in England,
the hundreds of thousands of plants, including the smallest mosses and
grasses, as well as the larger flowers, shrubs, and trees, the still
greater number of insects, spiders, snails, and larger animals and
birds, feeding on and hiding among them, and when we remember that in
the ever-warm tropical regions of the earth life is ten or twenty
times more exuberant than with us,–then the immensity of the living
population of the land and water of the globe becomes as difficult to
realize as are the figures in which the astronomer tells of the number
and distances of the stars. On the other hand, some idea of the number
of distinct species of animals and plants which have up to this date
been recognized and described by naturalists as at present existing,
may be formed by a statement of those which have been described in
some of the more familiar groups. About 10,000 species of mammals have
been described; about 14,000 of birds; 7000 of reptiles; 15,000 of
fishes; 500,000 of six-legged insects; 14,000 of crustacea (shrimps,
lobsters, crabs); 62,000 of molluscs (snails, mussels, etc.); 15,000
of star-fishes and sea-urchins; 5000 of corals and polyps; 3000 of
sponges; and 6000 of microscopic protozoa. In all about 800,000 species
of animals have been recorded, and probably as many more remain yet to
be recognized and described.
The total number of described species of plants has never been
estimated, but some idea of it may be formed from the fact that 1860
species of flowering plants alone have been distinguished in Britain,
17,000 in British India, and 22,000 in Brazil, not to mention those of
Africa and Australia! These figures do not include the vast numbers of
flowerless plants, the ferns, mosses, sea-weeds, mushrooms, moulds,
lichens, and microscopic plants.
And then we have to add to these enumerations of living species the
extinct species of successive geological ages, the remains of which are
sufficiently well preserved to admit of identification. Those which are
known are only a few thousands in number, and a mere fragment of the
vast series of species which _have_ existed in successive past ages of
the earth. They are a few samples of the predecessors of the existing
species, and some of them were the actual ancestors of those existing
to-day. The larger number of them have left no direct issue, but
represent side branches of the "tree of life" which have died out ages
ago.
[Illustration: STRANGELY-SHAPED FISHES.–1. The Coffer-fish (Ostracion);
2. Pteraclis, an oceanic fish allied to the so-called Dolphins; 3. The
Sun-fish (Orthagoriscus); 4. An Australian Blenny Patæcus.]