The Doctrine of Evolution: Its Basis and Its Scope

by Henry Edward Crampton

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Knowing the elements of the selective process, it is possible to analyze and to understand many significant phenomena of nature, and to gain a clearer conception of the results of the struggle for existence, especially when the human factor is involved. Let us see how much is revealed when the foregoing results are employed in a further study of some of nature's vital situations.

As a consequence of the many-sided struggle for existence, the interrelations of a series of species will approach a condition of equilibrium in an area where the natural circumstances remain relatively undisturbed for a long time. For example, among the field-mice of one generation, just as many individuals will survive as will be able to find food and to escape hereditary foes such as cats and snakes and owls. The number of owls, in their turn, will be determined by the number of available mice and other food organisms, as well as by the severity of the adverse circumstances that cause elimination of the less fit among the fledglings brought into the world. The vital chain of connections is sometimes astonishingly long and intricate. One remarkable illustration is given by Fiske, as an elaboration of an example cited by Darwin. He points out that the fine quality of the traditional roast beef of England is directly determined by the number of elderly spinsters in that country. The chain of circumstances is as follows: the quality of the clover fields, furnishing the best food for cattle, depends largely upon the visits to the clover-blossoms by wild bees, that accomplish the fertilization of the flowers by carrying pollen upon their bodies from one plant to another. Field-mice devour the young in the nests of these bees, so if there are few field-mice there will be many bees, and consequently better grazing for the cattle. The number of field-mice will vary according to the abundance of cats, and so the number of these domestic animals will exert an influence upon the whole foregoing chain of forms. But, as Fiske points out, cats are the favorite companions of elderly spinsters; therefore, if there are many of the latter, there will be more cats, fewer field-mice, more bees, richer clover fields, and finer cattle! Each link is real and the whole chain is a characteristic example of the countless ways that the natural destinies of living things are interrelated and intertwined.

The reality of such organic interrelationships is revealed with wonderful clearness in the numerous instances where some disturbing factor has altered one or another element of the balanced system. The invasion of the new world by Europeans has directly led to the partial or complete extinction of the tribes of Indians to whom the land formerly belonged; they have disappeared almost entirely from our state of New York, together with the bear and wolf and many other species of animals that formerly existed here. Wild horses and bison have also vanished before the advances of civilization and the alteration of their homes. Sometimes the extermination of one pest has resulted in an increase in the number of another through human interference with nature's equilibrium. In some of our Western states, a bounty was offered for the scalps of wolves, so as to lessen the number of these predatory foes of sheep. But when the wolves were diminished in number, their wild food-animals, the prairie dogs, found their lot much bettered, and they have multiplied so rapidly that in some places they have become even more destructive than the wolves.

One of the most remarkable illustrations is that of the rabbits introduced into Australia. This island continent was cut off from the surrounding lands long before the higher mammals evolved in far distant regions, so that the balance of nature was worked out without reference to animals like the rabbit. When the first of these were introduced they found a territory without natural enemies where everything was favorable. They promptly multiplied so rapidly that within a few years their descendants were numerous enough to eat up practically every green thing they could reach. Two decades ago, the single province of Queensland was forced to expend $85,000,000 in a vain effort to put down the rabbit plague. The remarkable statement has been made that in some places nature has taken a hand in causing a new type of rabbit to evolve. Finding the situation desperate, some of the animals have begun to develop into tree-climbing creatures. The animals exist in such numbers that the available food upon the ground is insufficient for all, and so some elimination results. But the young rabbits with longer claws, varying in this way on account of congenital factors, have an advantage over their fellows because they can climb some of the trees and so obtain food inaccessible to the others. If the facts are correctly reported, and if the process of selection on the basis of longer claws and the climbing habit is continued, the original type of animal is splitting up into a form that will remain the same and live upon the ground, and another that will be to all intents and purposes a counterpart of our familiar squirrel. All the evidence goes to show that squirrels have evolved from terrestrial rodents; if the data relating to Australian rabbits are correct, nature is again producing a squirrel-like animal by evolution in a region where the former natural situation has been interfered with by man.

The laws of biological inheritance have received close and deep study by numerous investigators of Darwinian and post-Darwinian times, because from the first it was clearly recognized that a complete description of nature's method of accomplishing evolution must show how species maintain the same general characteristics from generation to generation, and also how new qualities may be fixed in heredity as species transform in the course of time. Before our modern era in biology, the fact of inheritance was accepted as self-sufficient; now much is known that supplements and extends the incomplete account given by natural selection of the way evolution takes place.

It is not possible in the present brief outline to describe all the results of recent investigations, but some of them are too important to be passed over. Perhaps the most interesting one is that the laws of heredity seem to be the same for man and other kinds of living creatures, as proved by Galton and Pearson and many others who have dealt with such characters as human stature, human eye color, and an extensive series of the peculiarities of lower animals and even of plants.

The researches dealing with the physical basis of inheritance and its location in the organism have yielded the most striking and brilliant results. Darwin himself realized that the doctrine of natural selection was incomplete, as it accepted at its face value the inheritance of congenital racial qualities without attempting to describe the way an egg or any other germ bears them, and he endeavored to round out his doctrine of selection by adding the theory of pangenesis. According to this, every cell of every tissue and organ of the body produces minute particles called gemmules, which partake of the characters of the cells that produce them. The gemmules were supposed to be transported throughout the entire body, and to congregate in the germ-cells, which in a sense would be minute editions of the body which bears them, and would then be capable of producing the same kind of a body. If true, this view would lead to the acceptance of Lamarck's or even Buffon's doctrine, for changes induced in any organ by other than congenital factors could be impressed upon the germ-cell, and would then be transported together with the original specific characters to future generations. Darwin was indeed a good Lamarckian.

But the researches of post-Darwinians, and especially those of the students of cellular phenomena, have demonstrated that such a view has no real basis in fact. Many naturalists, like Naegeli and Wiesner, were convinced that there was a specific substance concerned with hereditary qualities as in a larger way protoplasm is the physical basis of life. It remained for Weismann to identify this theoretical substance with a specific part of the cell, namely, the deeply staining substance, or chromatin, contained in the nucleus of every cell. Bringing together the accumulating observations of the numerous cytologists of his time, and utilizing them for the development of his somewhat speculative theories, Weismann published in 1882 a volume called "The Germ Plasm," which is an immortal foundation for all later work on inheritance. The essential principles of the germ-plasm theory are somewhat as follows. The chromatin of the nucleus contains the determinants of hereditary qualities. In reproduction, the male sex-cell, which is scarcely more than a minute mass of chromatin provided with a thin coat of protoplasm and a motile organ, fuses with the egg, and the nuclei of the two cells unite to form a double body, which contains equal contributions of chromatin from the two parental organisms. This gives the physical basis for paternal inheritance as well as for maternal inheritance, and it shows why they may be of the same or equivalent degree. When, now, the egg divides, at the first and later cleavages, the chromatin masses or chromosomes contained in the double nucleus are split lengthwise and the twin portions separate to go into the nuclei of the daughter-cells. As the same process seems to hold for all the later divisions of the cleavage-cells whose products are destined to be the various tissue elements of the adult body, it follows that all tissue-cells would contain chromatin determinants derived equally from the male and female parents. As of course only the germ-cells of an adult organism pass on to form later generations, and as their content of chromatin is derived not from the sister organs of the body, but from the original fertilized egg, there is a direct stream of the germ plasm which flows continuously from the germ-cell to germ-cell through succeeding generations. It would seem, therefore, that the various organic systems are, so to speak, sister products in embryonic origin. The reproductive organs are not produced by the other parts of the body, but their cells are the direct descendants of the common starting-point namely, the egg. As the cells of the reproductive organs are the only ones that pass over and into the next and later generations, it will be evident, in the first place, that the germ plasm of their nuclei is the only essential substance that connects parent and offspring. This stream of germ plasm passes on in direct continuity through successive generations—from egg to the complete adult, including its own germ-cells, through these to the next adult, with its germ-cells, and so on and on as long as the species exists. It does not flow circuitously from egg to adult and then to new germ-cells, but it is direct and continuous, and apparently it cannot pick up any of the body-changes of an acquired nature. Now we see why individual acquisitions are not transmitted. The hereditary stream of germ plasm is already constituted before an animal uses its parts in adult life; we cannot see how alterations in the structure of mature body parts through use and adjustment to the environment can be introduced into it to become new qualities of the species.

It must be clear, I am sure, that this theory supplements natural selection, for it describes the physical basis of inheritance, it demonstrates the efficiency of congenital or germ-plasmal factors of variation in contrast with the Lamarckian factors, and finally in the way that in the view of Weismann it accounts for the origin of variations as the result of the commingling of two differing parental streams of germ plasm.