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 chang............