PRIMITIVE POLARITIES—HEREDITY AND VARIATION.
Heredity in simple forms of life—In more complex
organisms—Pangenesis—Varieties how produced—Fixed by law of
survival of the fittest—Dr. Temple’s view—Examples: triton,
axolotl—Variations in individuals and species—Lizards into
birds—Ringed snakes—Echidna.
As the earth is kept in an orbit, which makes life possible by the
balance of the antagonist centripetal and centrifugal forces, so is
that life evolved and maintained by the balance of the two conflicting
forces of heredity and variation. Heredity, or the principle which makes
offsprings resemble their parental organisms, may be considered as the
centripetal force which gives stability to species; while variation is
like the centrifugal force which tends to make them develop into new
forms, and prevents organic matter from remaining ever consolidated into
one uniform mass.
As regards heredity, the considerations which have been advanced in the
last chapter, on the origin of sex, will enable the reader to understand
the principles on which it is based. When a moneron, or living piece
of pure protoplasm, or its successor the nucleated cell, propagates
itself by simple division into two equal parts, it is obvious that each
half must, in its atomic constitution and motions, exactly resemble the
original. If amœba A divides into amœbæ B and C, both B and C are exact
facsimiles of A and of one another, and so are the progeny of B and C
through any number of generations. They must remain identical repetitions
of the parent form, unless some of them should happen to be modified by
different actions of their surrounding environment, powerful enough to
affect the original organisation.
In propagation by germs or buds, the same thing must hold true, only, as
the offspring carries with it not the half, but only a small portion of
the parental organism, its impress will be less powerful, and the new
organism will more readily be affected by external influences. When we
come to propagation by spores or single cells, and still more to sexual
propagation by the union of single cells of two progenitors, it becomes
more difficult to see how the type of the two parents, and of a long line
of preceding ancestors, can be maintained so perfectly.
Of the fact that it is maintained there can be no doubt. Not only do
species breed true and remain substantially the same for immense periods,
but the characters of individual parents and their ancestors repeat
themselves, to a great extent, in their offspring. Thus the cross between
the white and black varieties of the human species perpetuates itself
to such an extent, that a single cross of black blood leaves traces for
a number of generations. In the Spanish American States and the West
Indies, where the distinction is closely observed, the term ‘octoroon’
is well known, as applied to Creoles who have seven-eighths of white
to one-eighth of black blood in their composition. In the case of what
is called ‘atavism,’ this recurrence to the characters of ancestors
is carried to a much further extent. In breeding animals, it is not
uncommon to find the peculiar features of generations of ancestors
long since extinct cropping up occasionally in individuals. Thus,
stripes like those of the ass along the back and down the shoulders,
occasionally appear on horses whose immediate ancestors for many
generations back showed nothing of the sort; and even stripes across
the legs like those of the zebra occur quite unexpectedly, and testify
to the common descent of the various species of the horse tribe from a
striped ancestor. How these ancestral peculiarities can be transmitted
through many generations, each individual of which originated from a
single microscopic cell which had been fructified by another cell, is
one of the greatest mysteries of nature. It may assist us in forming
some idea of the possibility of a solution to remember what has been
proved as to the dimensions of atoms. Their order of magnitude is that
of a cricket-ball to the earth. In a single microscopic cell, therefore,
there may be myriads of such atoms circling round one another and forming
infinitesimal solar systems, of infinite complexity and variety. Darwin’s
theory of ‘Pangenesis’ supposes that some of the actual identical atoms
which formed part of ancestral bodies are thus transmitted through their
descendants for generation after generation, so that we are literally
‘flesh of the flesh’ of the primæval creature who was developed into man
in the later tertiary or early glacial period. Haeckel, more plausibly,
suggests that not the identical atoms, but their peculiar motions and
mode of aggregation have been thus transmitted: a mode of transmission
which, with his prevailing tendency to invent long and learned names
for everything, he calls the ‘Perigenesis of plastids.’ In any case,
however, these must be taken not as solutions of the problem, but as
guesses at the truth which show that its solution is not impossible.
The opposite principle to heredity, that of variation, is equally
important and universal. It is apparent in the fact, that although
every individual of every species reproduces qualities of parents and
ancestors, no two individuals do so in precisely the same manner; no two
are exactly alike. This difference, or individuality, becomes more marked
as the organism is higher. Thus, sheep and hounds differ from one another
by slight differences which require the practised eye of the shepherd
or huntsman to detect; while human beings are so unlike, that of the
many millions existing in each generation no two exactly resemble one
another. The reason of this is apparent if we consider that the higher
the organism the more complex does it become, and the less the chance of
the whole complicated relations of parent and ancestral organisms being
transmitted by single cells so solidly and completely as to overpower
and remain uninfluenced by external influences. Variation evidently
depends mainly on the varying influences of environment. If the exterior
layer of molecules of a lump of protoplasm become differentiated from
the interior ones and form a cell-wall, it is because they are in more
immediate contact with the air or other surrounding medium. Internal
changes depend on conditions such as temperature and nutrition. In the
case of cultivated plants and domestic animals we can see most clearly
how varieties are produced by adaptation to changes of environment. These
variations, however, would not proceed very far, were it not for the
interaction of the opposing forces of variation and heredity, by which
latter the variations appearing in individuals are fixed and accumulated
in descendants, until they become wide and permanent divergencies.
This is done in the case of cultivated plants and domestic animals by
man’s artificial selection in pairing individuals who show the same
variations; and in nature by the struggle for existence, giving victory
and survival to those forms, and in the long run to those forms only,
whose variations, slight as they may be in each generation, tend to bring
individuals into better adaptation to their environment.
It is the great glory of Darwin to have established this firmly by an
immense number of interesting and exhaustive instances, and thus placed
evolution, or a scientific explanation of the development and laws of
life, on a solid basis. Every day fresh discoveries and experiments
confirm this great principle, and it has almost passed into the same
phase as Newton’s law of gravity, as a fundamental law accepted as
axiomatic by all men of science, and as the basis of modern thought, to
which all religions and philosophies have to conform, accepted by nearly
all modern thinkers. I may here quote a passage from an eminent Anglican
divine, Dr. Temple, for the double purpose of showing how universal
has become the acceptance of this Darwinian view of evolution among
intelligent men; and how little terrible are its consequences, even to
those who look at the facts of the universe through a theological medium
and retain their belief in accepted creeds.
‘It seems in itself something more majestic, more befitting of Him to
whom a thousand years are as one day, and one day as a thousand years,
thus to impress His will once for all on this creation, and provide for
all its countless varieties by this one original impress, than by special
acts of creation to be perpetually modifying what He had previously
made.’[1]
[1] Dr. Temple, _Religion and Science_.
Scientific men would be content to accept this statement of Dr. Temple’s
almost in his own words, except that they might consider his definition
of the Great First Cause as somewhat too absolute and confident. Having
had to deal so much with actual facts and accurate knowledge, they
are apt to be more modest in assertion than even the most enlightened
theologian, whose studies have lain rather in the direction of phrases
and ideas, which, from their very nature, are more vague and indefinite,
and perhaps rather guesses and aspirations after truth, than proofs of
it. In any case there is the authority of a learned and liberal-minded
bishop for the position that the scientific way of looking at the
universe is not necessarily profane or irreligious.
To return to variation: the instances of the operation of this principle,
alone or in conjunction with that of heredity, in working out the
evolution of species, are exceedingly numerous and interesting. Those who
wish to understand the subject thoroughly must study the works of Darwin,
Haeckel, Huxley, and other modern writers; but for my present purpose it
will be sufficient to refer to a few of the most marked instances which
may assist the reader in comprehending how the gradual evolution of life
and creation of new species may have been brought about.
There is an amphibious animal, called the triton or water-salamander,
akin to the frog, whose normal course is to begin life living in the
water and breathing by gills, and end it on land with gills metamorphosed
into lungs. If they are shut up in water and kept in a tank they
never lose their gills, but continue through life in the lower stage
of development, and reproduce themselves in other tritons with gills.
Conversely the axolotl, a peculiar gilled salamander from the Lake of
Mexico, has its normal course to live, die, and propagate its species in
water, breathing by gills; but if an axolotl happens to stray from the
water and take to living on dry land, the gills are modified into lungs
and the animal gains a place in the class in the school of development.
This fits in remarkably with the fact that the embryo of all vertebrate
mammals, including man, passes through the gilled stage before arriving
at the development of lungs, which assists us in understanding two facts
of primary importance in the history of evolution.
First, how terrestrial life may have arisen from aquatic life by
adaptation to altered conditions.
Secondly, how the evolution of the embryo sums up in the individual, in
the period of a few days or months, the various stages of evolutions
which it has taken millions of years to accomplish in the species.
As a parallel to the transformation of gills into lungs, and of an
aquatic into a land animal, if we turn to the geological records of the
Secondary period we may trace the transformation of a water into an air
population, of sea-lizards into flying-lizards, and of flying-lizards
into birds. The ‘Hesperornis’ is an actual specimen of the transition,
being a feathered lizard, or rather winged and feathered creature which
is half lizard and half bird.
A remarkable instance of the great change of functions which may be
produced by a change of outward conditions is afforded by the common
ringed snake, which in its natural state lays eggs which take three
weeks to hatch; but if confined in a cage in which no sand is strewed
it hatches the eggs within its own body, and from oviparous becomes
viviparous. This may help us to understand how the lowest order of
mammals, which, like the Australian echidna or duck-billed mole, lay
eggs, may have developed, first into marsupial, and finally into
placental mammals.
These examples may assist the reader in understanding how the infinite
diversities of living species may have been developed in the course of
evolution from simple origins, just as the inorganic world was from
atoms, by the action and reaction of primitive polar forces between the
organism and its environment, and between heredity and variation.