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 am?ba a divides into am?b? b and c, both b and c[118] 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[119] 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 prim?val 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[120] ‘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[121] 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[122] 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[123] 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[124] 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.
