HOW ANIMALS ADAPT THEMSELVES TO CIRCUMSTANCES
We all know what it is to adapt ourselves to circumstances. Suppose two
lads, fresh from school, go out into the world to earn their living;
one becomes a navvy and one a clerk. In five years' time these two young
men will probably be very different in appearance from one another.
The navvy will have developed his muscles; he will be broad-built,
broad-chested, and strong. The clerk, on the other hand, will probably
be comparatively weak and slim, his chest will not be so broad, his
muscles will not be so well developed. The navvy, too, will probably
be of a fresh complexion, while the clerk will be pale. All these
differences are due to the fact that their bodies have adapted
themselves to circumstances. Both men may be equally healthy, and
equally long-lived. Let us take another example. Let us compare two
other youths, of whom one becomes a cobbler and one an Alpine guide.
The latter, in five years' time will have become a perfect specimen
of muscular humanity--active, agile, and hardy. The cobbler will be
comparatively stiff in his limbs and unable to undertake any singular
feat of muscular exertion, although he may be able to do a very hard
day's work at his own trade. The mountaineer, too, will probably differ
in disposition from the cobbler. He will be daring, resourceful, and not
afraid of danger under circumstances which would terrify the cobbler.
Now let us suppose that the sons and grandsons of the navvy are brought
up to be navvies, and the sons and grandsons of the clerk are brought up
to be clerks;--that the children and grandchildren of the Alpine guide
follow his own calling, and the children and grandchildren of the
cobbler do the same;--we shall probably have four families differing
very much in type of physique from one another. Yet take one of the
navvy's sturdy grandchildren and bring him up as a clerk, and he will
lose much of his sturdiness. Let the mountaineer's grandsons be brought
up as cobblers, and by the time they are thirty they will not be
remarkable for their muscular capabilities.
Just in a similar way the bodies of animals adapt themselves to
circumstances. It is not always possible to trace the steps by which
this has been done. But sometimes it is so; and we may find a whole
series of varieties that are plainly due to adaptation. When we see an
animal which is in some way especially fitted for its surroundings, we
are therefore justified in concluding that it has become so by degrees.
The way in which animals adapt themselves to their surroundings in the
matter of colour would afford material for several volumes each as large
as this one. Those who have not travelled in foreign countries may
perhaps find it difficult to realise that brilliant colouring and showy
patterns can ever enable an animal to hide itself successfully. But an
instance may be taken from an animal common on our own shores which will
illustrate how this principle works.
In the spring there may be found in large numbers upon our rocky coasts
a little oval shell-fish, about one-third of an inch long, sticking to
the fronds of the tangle and other broad-leaved seaweeds. The animal is
of a very pale brown colour; its shell brownish and semi-transparent,
with several stripes of brilliant turquoise blue down the back. These
stripes are not continuous, but interrupted at intervals so as to give
them a beady look. Taken in the hand and looked at closely, the shell,
with its contrast of blue stripes on a brown ground, is extremely
conspicuous; brown being, in fact, the contrast-colour which shows blue
in its greatest brilliancy. Yet, when perched upon the tangle, the
creature is almost invisible, and might easily be mistaken for a natural
irregularity of the surface of the seaweed. While the brown is the
colour of the seaweed itself, the brilliant blue is indeed the exact
colour of the spring sky at that season, everywhere reflected from the
sea-water and from the wet surface of the seaweed. By matching that
brilliant colour the animal therefore is rendered invisible. This
little creature is the young of the Semi-transparent Limpet, _Patella
pellucida_. This, at least, was the old-fashioned name for it, though
it has received others. Its young and its adult form are so different
in the appearance of the shell, that they have been described under
different names. English readers who search for it in the spring will
learn by experience that bright colouring may help to make a creature
invisible. But this is not all that is to be said about the protective
colouring of this little shell-fish. There are many creatures whose
young live at the surface of the sea, and afterwards migrate to deeper
water as they attain adult age. In early life they are transparent,
because thus they best escape notice in the clear water of the surface,
especially when seen from below, by the many enemies on the watch to
devour them. But in their later life they become opaque, because thus
they best escape notice from enemies watching from above, as they crawl
along the bottom of the sea. Now this is the case with the little
Patella. For this also migrates to the bottom--in this instance a
comparatively short journey--when it is ready for adult life. Both
shell and animal, therefore, are at first nearly transparent, but in
older life both become more opaque; the blue stripes, too, are almost or
quite obliterated in the after-growth of the shell, slight traces of
them alone remaining at its apex. This change of colour fits the animal
for the new home in which it settles, for it moves down from the leaf
of the tangle to its root, and there finds a snug shelter among
the coral-shaped branches of which the root is composed. Not many
reflections of the blue sky are likely to reach the recesses of the
tangle-root, so the creature has no longer any need of its protective
colouring of blue.
The adult shell, however, retains a certain degree of translucency,
which matches very well with the colouring of the tangle-root; and thus
presents a great contrast to the shell of the common Limpet, which is
found on rocks. The rugged surface of the latter is usually more or less
irregularly speckled in harmony with the surfaces on which it lives,
though this shell also presents when young occasional touches of blue,
which suggests a family likeness in colour tastes on the part of the two
kinds of Limpet. The blue in this case, however, is of the dullest and
dingiest shade. The _Patella pellucida_ is common on the more rocky
portions of our coasts; in spring the young may be seen in thousands on
the seaweeds of the Isle of Man; here its habits were first observed and
described in detail by the Manx naturalist Forbes, who noticed its
peculiar way of finding a hiding place among the roots of the tangle.
The same shell-fish, in contrast with the commoner Limpet of the rocks,
affords another instance of the way in which shells adapt their forms
to their surroundings. In each case the shell is a plain conical cap,
and the animal within keeps the shell firmly attached to the base on
which it rests. The Limpet can move about at a very creditable snail's
pace when it wishes to do so, and at low-water mark, when the tide is
beginning to rise, you may easily find them moving about and off their
guard; but during many hours of the day, when the tide is out, the main
object of the Limpet is to keep its shell as firmly fixed to the rock as
possible. It will at once be seen that if the margin of the shell were
smooth like that of a tea-cup, and the surface of the rock to which it
clung very irregular, many chinks would be left between the margin of
the shell and the surface of the rock through which unwelcome visitors
might find entrance. The loss of moisture through the crevices, too,
would be a serious thing to the animal during the hours when the shell
is uncovered by the tide and exposed to the rays of a hot sun. On the
other hand, if the margin of the shell were irregular, and the surface
on which it rested smooth, unprotected crevices would in the same way
be left. So the Limpets adapt the shape of their shell to their
surroundings; the _Patella pellucida_, which lives on the smooth
branches of the tangle-root, has a shell with a smooth regular edge;
while the _Patella vulgata_, which lives upon rocks, has a shell with an
irregular, indented edge, whose irregularities fit into those of the
rock on which it rests. (See Fig. 2.)
[Illustration: FIG. 2.--Shells mentioned in Chap. II. 1, Common Limpet,
old and young; 2, Semi-transparent Limpet, old and young (the remains
of the young shell may be seen crowning the adult shell); 3, Common
Yellow Periwinkle; 4, Common Edible Periwinkle; and 5, High-tide-mark
Periwinkle, both with a sharp spire, for comparison. One specimen of
the latter stands among group 3.]
Probably every reader will be able to appreciate the above instances of
creatures adapted to their surroundings. For there are few people who
are not familiar with the common Limpet of the shore between tide-marks,
and with the great seaweed called Tangle, which has its habitat a little
lower down, and forms great sea-meadows, whose upper limits alone are
ever laid bare by the tide. The _Patella pellucida_, too, is fairly
common, and the dead shell may be found on most rocky parts of our coast
all the year round. As for the blue-striped young shell, floating on the
blades of the tangle, those who have leisure to visit the seaside during
the months of spring and early summer, may have seen it as I have
described it; and the mention of it will recall pleasant memories of
clear spring skies, and fresh sea-winds, and fields of heavy tangle
swaying gently on the swell that comes in from the open sea. It is
interesting to know something of the habits of the creatures whose forms
we study, and we have already spoken of the snug little hiding-place
that the Semi-transparent Limpet finds for itself in the tangle-root.
It is of interest to remember that the Common Limpet, too, is a
home-loving creature, which knows and prefers the spot of rock on which
it habitually rests; and can find its way back to it, aided by its two
eyes and two smelling patches. This has been proved by Professor Lloyd
Morgan, who has recorded the result of his observations, made on the
coast of Dorsetshire. It is not easy to detach a Limpet from the rock
without injuring or exhausting it, but these specimens were caught when
moving of their own accord, and were therefore uninjured and brisk. They
were removed to short distances, and the following table shows the
result of the experiment, clearly proving that the Limpet prefers home,
but regards a distance of two feet as a very long journey.
----------+-------------+----------+---------+--------
| | Number | |
Number | Distance in | Returned | In Four | Later.
Removed. | Inches. | in Two | Tides. |
| | Tides. | |
----------+-------------+----------+---------+--------
25 | 6 | 21 | 0 | 0
21 | 12 | 13 | 5 | 0
21 | 18 | 10 | 6 | 2
36 | 24 | 1 | 1 | 3
----------+-------------+----------+---------+--------
Similar observations were made at an earlier date, by Mr. George
Roberts, at Lyme Regis.
Let us now take an instance of adaptation in form. And this time we will
take a shell so common that everybody will know it.
Everyone who has spent a little time in naturalising on the shore, has
noticed how often you may find univalve shells, such as those of the
whelk and periwinkle, with the top of the shell knocked off. This is
nearly always the case with the dead shells that you find strewn along
the tide-line; and after a storm, on a rocky coast, you may find shells
that still contain the living tenant, in the same sad condition. And you
may also meet not infrequently with shells, dead or living, that bear
evidence of the owners' efforts to repair them after an accident to the
spire. A piece has been broken, and you find it cemented on again by a
patch of shell, serviceable no doubt to the owner, but crooked and
unsightly in appearance. Now there is a very common shell, the little
yellow periwinkle, which has practically done away with its spire, the
coils of the shell being so curved that the earlier part of the spire
does not project beyond the later-formed coils, and the whole shell has
a rounded outline. This little creature lives on the long seaweeds which
grow at low-water mark or near it; and when the sea is rough it is
obviously liable to be dashed from its foothold on the seaweed and flung
violently down, as the huge seaweeds sway about in the shallow waves. We
may easily satisfy ourselves that this is an accident that frequently
happens, by examining the shore when the tide is going out, on some
stormy spring or autumn day. Numbers of the yellow periwinkles are then
to be found crawling on the sand, and striving to regain their place in
the seaweedy rocks as soon as possible. On a calm day you will rarely
see one crawling on sand above low-water mark, for it is a place they do
not choose by preference; those that are to be found there on the stormy
day have lost their foothold, and have been washed about by the tide.
Had they, like some other kinds of periwinkle, a sharp spire, how many
would be the casualties under these circumstances! But as it is, you do
not see a single specimen with a broken top: the rounded spire is an
adaptation to circumstances, required for the protection of the tenant
of the shell. (See Fig. 2.)
It may be added that the yellow Periwinkle is not only protected from
mechanical sources of danger by its form, but is also in some degree
protected from living enemies by its colour. This, at first sight, seems
exceedingly conspicuous. We must remember, however, that the animal
often lives in that part of the shore where the Bladder Seaweeds, or
Fuci, are extremely abundant. The flowering ends of these are of a
yellow colour, fairly bright. When seen from below, with the sunlight
streaming through them, they no doubt appear much brighter than when
seen, as we see them, from above, with the sunlight falling on them.
Now protection from foes below is what the yellow periwinkle needs
most: for fishes are quite ready to swallow it whole, and are not in
any way deterred by the thickness of the shell, which is (by-the-way)
in a measure a protection against birds when the tide is out; fishes
habitually swallow shell-fish whole, although the inmate only is
digested. The bright yellow, then, that seems to us so conspicuous, is
probably a good means of hiding for the periwinkle when under water. Its
common variations in colour, too, are probably protective in their use:
some are a dull purplish brown, some drab. These are good colours in
which to lie hidden, respectively, under darker tracts of seaweed, or
upon the rock itself. This little shell is so abundant on rocky coasts
that on some beaches the dead shells are as numerous as pebbles. No
wonder, with all these adaptations for protection!
Another instance of adaptation to circumstances is described in the
sea-urchin shown on p. 125. This is one among many instances where
animals that live on sand or mud acquire a flattened shape, so that
their weight is distributed, and the danger lessened, of their sinking
in a quick-sand. The flat-fish, such as soles and flounders, are a
familiar example; and the same principle is illustrated by the flattened
forms of many of the bivalve shell-fish, whose flat shell, when closed,
can lie safely on the loosest sand. Equally is their form adapted for
their circumstances, when, in their slow way, they begin to move. For
the flat valves of the shell are placed to the right and left of the
animal's body. So that when it stirs, or floats quietly in the current
of the tide, the shells present their sharp edges to the resistance of
the water, thus enabling the creature to move like a ship through the
sea, or like a knife-blade through bread, with the least possible
friction: and specially is this provision for the lessening of friction
important, when we consider that many of these bivalve shell-fish have
to move, not only through water, but also through sand and mud.
It may be assumed that every reader is familiar with the common forms of
the bivalve shell-fish. The frontispiece shows one of them, considerably
flattened in shape.
So far, however, we have not explained _how_ animals adapt themselves to
circumstances; we have only pointed out the fact that they do so.
Take the case of our little Limpet. It cannot say: "I will paint myself
with blue and brown, so as to be mistaken for a bit of seaweed
reflecting the blue sky"; nor can the periwinkle say: "I will paint
myself with yellow, so as to pass unnoticed among the yellow ends of the
_Fucus_; and I will build my spire low, so that it will not be broken."
The bivalve shell-fish and the Sand-Cake sea-urchins do not say to one
another, "Let us alter our shells, and build them a little flatter, so
that we shall not sink in too deep when we lie upon the ooze and sand of
the sea."
How then do these adaptations take place? Darwin has explained this for
us. Individuals often have some little peculiarity, in which they differ
from the average of their kind. The establishment of such little marks
of individuality is spoken of as Variation. If among these individual
peculiarities there is one which is in any way disadvantageous, _e.g._
one which tends to make the creature conspicuous in the sight of its
foes, the owner will be quickly eaten, and of that peculiarity there
will be an end. If, on the contrary, the peculiarity gives the owner
some advantage over its fellows, that individual will survive, and
probably transmit its peculiarity to some of its descendants.
We have seen, for instance, that it is of advantage to our little
periwinkle to be yellow, when it lives in certain situations; and that
it sometimes presents other colours, likely to be favourable in other
cases. If we gather together a large number of specimens, we shall find
a surprising range of variation in colour. Some present a tint of bright
orange, nearly red; some are a dull brown; the dark purple shade and the
drab have been already referred to. The very young shell usually
presents an unmistakable shade of pink; and we may find innumerable
half-grown specimens in which we may trace the gradual establishment of
the advantageous yellow colour, from an original shade of unmistakable
pink, presented by the earlier whorls. Kindred varieties of the shell,
too, may be found with stripes or speckles. Since this very common shell
may be found in abundance on any rocky shore in the British Isles, the
reader may easily study its colour-variations, both in the dead and the
living shell. Study also the ground on which the creature lives, with
its sharp colour-contrasts of rock and seaweed patches, and it will be
easy to understand why the colours are thus varied, with a
preponderance, on the whole, of the yellow shades. It is all a question
of the survival of the fittest--the unfit being represented by colours
too easily seen, and therefore quickly snapped up. As for the spire, it
has already been shown how that is adapted to circumstances. It is
worthy of remark that in the kindred Edible Periwinkle, _Littorina
littorea_, which has a sharp spire, elderly specimens may be seen with
the end of the spire damaged.
Turn again for a moment to our first instance--the adaptation of men to
a sedentary or an outdoor occupation. Here we dwelt upon the change
produced by their mode of life; we left out of sight the "survival of
the fittest." Yet here it is equally surely at work. How often does the
young mountaineer, less agile than his fellows, come by a violent death?
Only those who are equal to the necessities of the life survive--many
are lost. How often does the clerk, tied to his desk, fail in health
and die? How often, hating a sedentary life for which he is unfitted,
does he throw his energies into athletics, lose interest in his office
work, and get dismissed? Here again comes in "the survival of the
fittest"--for a desk: alas! perhaps the only means of livelihood.
But _why_ do variations occur? This is the question first asked by a
child, when you try to explain the working of "natural selection." It is
also the last question asked by scientists, who are still industriously
engaged upon studying the problem.
In the above instances from human life, we have considered the
occurrence of changes brought about in the organism by the circumstances
of life; or as scientists say, by the "environment." Scientific men are
busily hunting for instances of variation of this sort. Take for
example, an animal which lives sometimes in salt water, sometimes in
water that is only brackish; there are cases in which small differences
can be noticed, according to the difference in the habitat. Notice the
marine shell-fish, for instance, near the estuary of a river: they are
often less robust specimens than are found at a point free from the
influence of fresh water.
Not until the effect of known causes on the rise of variations has
been studied much more fully than at present, will it be possible to
judge regarding the nature of those variations which _appear_ to be
spontaneous; for which, at present, no predisposing cause can be
assigned.
A very large number of variations, however, fall into the class of
"Atavistic" variations; that is to say, those which show a return to an
ancestral type. These are variations which are very rarely welcome. If,
for instance, a boy has a pair of handsome black rabbits, he is not much
pleased to find among their progeny, every now and then, one of the
colour of the original wild Bunny. The probability, in this case, is
that the atavistic variety will find its way into a pie, instead of
being kept as a pet. Equally unsatisfactory to the owner, is the
incorrigibly savage and intractable dog or horse--a reversion to the
mental type of an ancestor which knew not the authority of a master.
Atavistic variation often occurs when members of two well-marked
varieties are mated; so that in some of the offspring produced, each
parent seems to cancel out the more extreme characteristics of the
other, leaving only the characteristics of the more generalized
ancestral type, from which both parents have alike been derived.
When the ancestral type is in some way inferior to the modern one,
variation which consists in reverting to the former is often referred to
as Degeneracy. There is reason to believe that discomfort and hardship
of existence tend to produce variation of this kind--a fact of supreme
importance, when the problem of Degeneracy is considered in connection
with human life. When creatures begin to degenerate, it is, in fact, as
if the species were saying to itself, "I have gone astray; let me
retrace my steps along the road by which I came, and maybe I shall find
comfort and safety; step by step I will try to go back to my ancestral
form."
Very rapid variation of any sort is indeed often a sign that the
struggle for existence is too hard for the type in question. The
palæontologist can tell us of types that present numerous variations
before becoming extinct; while others, comfortably holding their own in
the struggle for existence, remain practically unchanged during age
after age of the geological record, and survive even up to the present
day. We may borrow from commercial life a homely illustration that will
explain this aspect of variation. When competition in trade is keen, the
seller must have novelties; he will try all sorts, and find some good,
some bad, some indifferent. If he now revives an out-of-date pattern of
goods, for the sole sake of change, this is Degeneracy. But where, on
the contrary, competition is dull, the same firm will turn out the
same goods for a long period of time. There is an optimum in trade
competition: a reasonable competition results in the production of
sensible novelties, and consequent progress; but competition over-keen
results in the production of rubbish, leading to eventual failure. So in
the world of animal life; a certain degree of struggle for existence
results in variation, establishment of new varieties, progress. A
greater degree results in too rapid variation, new varieties that
speedily perish, and finally, the extinction of the type.
We have spoken of "varieties." Each of the domestic animals presents
varieties, which are the cumulative result of the breeder's artificial
selection of natural variations. Thus the Pug and the Collie for
instance, are varieties of the Dog; the Bantam and the Dorking of the
Fowl. Among wild animals, varieties are similarly produced by _natural_
selection, resulting from the "survival of the fittest." By degrees,
intermediate forms are lost; and new species are established by the
greater and greater divergence of varieties originally derived from one
ancestral type.
TABLE SHOWING THE POSITION IN CLASSIFICATION OF THE ANIMALS NAMED IN THE
FOREGOING CHAPTER
=Phylum= =MOLLUSCA=, or Shell-fish.
=Class= GASTEROPODA, or Snail-like Shell-fish.
=Sub-Class= ANISOPLEURA, or Unequal-sided Gasteropods.
=Branch= STREPTONEURA, or Unequal-sided Gasteropods with nerves
twisted into the shape of a figure of 8.
/------------^-----------\ /--------------^--------------\
=Order= ZYGOBRANCHIATA, AZYGOBRANCHIATA,
or Streptoneura with or Streptoneura with
a pair of gills. only one gill.
=Genus= _Patella_, the Limpet, _Littorina_, the Periwinkle,
with gills obliterated, or Shore Shell.
and only indirectly
represented; breathing
is performed by folds
of the mantle.
=Species= _Vulgata_, the Common _Littoralis_, the (Yellow)
Limpet. Periwinkle that lives
above low-tide-mark.