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THE PROCESS OF EVOLUTION

TH

HE evidences of organic evolution are so numerous and conclusive that it is no longer justifiable to speak of this conception as a theory. It is a fact which no one who has investigated the case can doubt or deny. The evidence is available from many sources-the record left by fossils in successive layers of rock; the peculiarities of geographic distribution; the course of development of the individual; the existence to-day of forms representing innumerable stages along the path travelled by the most advanced animals and plants, as confirmed by fossils; and relationships among all living things based on descent from common ancestors, as evidenced by structure and function, and, in animals, by blood tests as well. These subjects need only be mentioned to recall to the biologist such an array of evidence as to place a presentation of the case in the same category as, for instance, an exposition of the shape and movements of the earth. If there are still some who refuse to face the facts in one or both of these cases, it cannot be helped.

But, if the fact of evolution has been established, there is doubt as to its method, an uncertainty which has recently developed as a result of experiments whose results tend to disprove Darwin's theory of the causal factors in evolution. This modern doubt has caused some people to think that the 'theory' of evolution itself had been discredited, whereas nothing could be further from the truth. It may be appropriate, therefore, to review the theories about the way in which evolution has occurred, and to state the present attitude in this respect.

The idea that all living animals and plants have evolved from more simple forms of life, and originally have come from some common primitive ancestors was not original with Darwin. It was discussed by Aristotle and was repeatedly stated thereafter (and doubtless before also) by independent thinkers who observed a world of change and evidences of relationships, and conceived this profound idea of common

origin and continued variation. The question by what means these variations came about was answered about the beginning of the nineteenth century by Lamarck, who claimed that the changes wrought on the individual by his environment are transmitted to his offspring. Thus these changes are cumulative and result in a variety of creatures admirably adapted to the conditions in which they live. Lamarck's work was highly speculative and in certain respects so absurd as entirely to discredit his whole theory with most people. Although he .is to-day considered one of the great figures in the history of biology, he died in a Paris garret in extreme poverty and neglect. The familiar lines from Lowell's Biglow Papers refer to the more extreme aspect of his theory:

Some flossifers think that a fakkilty's granted,
The minnit it's proved to be thoroughly wanted,

Thet the fears of a monkey whose holt chanced to fail,
Drawed the vertibry out to a prehensile tail.

But the idea did not die. The grandfather of the great Charles Darwin held somewhat similar views; and a modified theory of the inheritance of acquired characters, as it is known, is the subject of controversy in the scientific world to-day. However, this explanation of the process of evolution was not acceptable; it was not supported by indisputable observation or experiment, and the condition of apparently fixed species was not consonant with a thorough-going conception of the inheritance of acquired characters. Moreover, the orthodox belief was in special creation rather than in evolution. It remained for Charles Darwin in 1859 to submit so great a mass of observations that his conclusion, that evolution had been the method of creation, could not be escaped. He went further, and this indeed was his great contribution. He offered what appeared to be a reasonable explanation for evolution.

Every living creature, he said, is producing offspring that varies about a type similar to itself. The variations are heritable, and are fluctuating, quantitative and continuously occurring. He did not state their cause, although later he approved of a moderate Lamarckism. Every kind of plant and animal is also reproducing at so great a rate that its environment will

permit only a small fraction of its progeny to persist. Those best suited to their surroundings will succeed in the 'struggle for existence', and will be able to reproduce their kind. The others will eventually die out. This is 'natural selection', or the 'survival of the fittest.' Favourable variations will thus be selected by nature for survival, unfavourable for disappearance. Thus, gradually, divergence and diversity of forms occur, giving rise to a complex system of forms of life all very closely fitted to their environment, and still throwing off varieties of individuals. Most of these are doomed to early death, but some may suit their environment still better, or reach other situations where they may persist.

The evidence adduced by Darwin as to the fact of evolution does not require a detailed recital here. No intelligent person who has investigated the subject can reach any other conclusion. Darwin swept the scientific world of his day, prepared as it was by the state of knowledge at that time, and it did not take long for the theory of evolution to reach the rank of orthodoxy. To-day, much more evidence is available, including more extended observation along the lines of Darwin's work, and through such later work as the remarkable confirmation of animal relationships demonstrated through blood tests. It is perhaps needless to say that man is thus proved to be also a product of evolution from the common stock of life. The diversity that is evident both within and among races of men in morphological details, in mental ability, and in every other inherited characteristic, is strikingly in accordance with expectations based on this theory.

But if the fact of evolution is undisputed, theories as to its method are not. Darwin's idea that species are built up by natural selection, acting or fluctuating, quantitative and continuous variations is the essence of Darwinism, which term will henceforth be used with that connotation, although the popular conception of the name is the theory of evolution itself. Darwin confined himself chiefly to observation, and did not perform any intensive breeding experiments to test his theory. Now, nobody denies that variation is a universal characteristic of life. No two individuals of any given kind of creature are precisely alike. The problem of testing Dar

winism therefore lies in determining the extent of inheritance of parental variations and the degree to which these variations go. Can species really originate in this way? If so, why cannot they be reproduced at will, and evolution be observed in progress like water flowing?

This problem was undertaken by many workers, and led to the discoveries of de Vries at the beginning of the present century, and his new theory of mutations, or discontinuous, constant and qualitative variations differing in every respect from those of Darwin. De Vries had been unable, by breeding, to get very far with the fluctuating variations observed by Darwin, and he was convinced that they are not sufficiently progressive to produce a new species. (Species are notoriously stable, differ from each other by distinct morphological characters, and usually cannot be crossed). But he did find sudden, distinct variations, which breed true, being thrown off by an evening primrose - Oenothera lamarckiana. He called these variations mutations, and proposed a new theory which accounted for the genesis of new species by the throwing off of mutations at intervals by all forms of life.

Just as Darwin, in publishing his theory of the origin of species, provoked intensive investigation and thought, which has confirmed the theory of evolution, and to a great extent natural selection, but has lately discredited his idea of variations, as will presently be shown, so also in but a partial way has de Vries's mutation theory been accepted. An extraordinary amount of work has been done during the present century in the experimental study of heredity, or genetics, as the new science is called. It began with the rediscovery by de Vries and two others simultaneously of the fundamental principles of heredity originally worked out by Gregor Mendel, a monk in Brünn or Brno, in what is now called CzechoSlovakia, and published by him in 1865. This work never came to the attention of Darwin, was unappreciated by those who saw it, and was virtually lost, until found by de Vries after he himself had come to the same conclusions. Mendelism has since advanced far beyond the conceptions of Mendel, and, as a result of that and of other discoveries, we can now say that mutations do occur, but that the 'mutations' de Vries

discovered and on which he founded his theory, are not mutations at all, but mainly the effects of Mendelian segregation although of a peculiar sort.

Mendel's work was done with various races of garden peas, and his results have since been confirmed with innumerable kinds of plants and animals. It is more convenient to state as understood to-day the principles which he discovered. All the inherited characters of an individual are said to be due to determiners or genes for these characters located on the chromosomes in each cell of which the animal or plant is composed. As to the physical nature of the gene nothing is positively known. It is really a hypothetical unit used in the same way as atoms and molecules by the physicist. From their reactions we know they must be ultra-microscopic in size and it is thought that they may be single molecules of complex proteins. The chromosomes, along which the genes are located in a row, are the important separate bodies which occur in pairs in body cells, and singly in germ cells, and together constitute the nucleus, the vital part of that structural unit of life, the cell. The number of chromosomes is definite for a species, and ranges from one pair in a parasitic worm, to hundreds in some plants and animals. There are 24 pairs in the human race.

Individuals which breed true for a certain character have a gene for it in double dose, i.e., the gene is present on each of the pair of chrosomes concerned. On formation of germ cells (eggs or sperms) the chromosomes are reduced to one of each pair, so only one gene for the character in question is present in an egg or sperm. On fertilization, or the fusion of this egg or sperm with a sperm or egg from another parent, the pairs are restored. If this other parent breeds true for a contrasting character, such as tall and dwarf in plants, or blue eyes and brown in man, then the new individual will have a gene for each. If these contrasting genes are of equal potency, a blend will result, as is the case in mating black Andalusian fowls with white, which gives blue. But usually one expresses itself or is dominant, and the other is concealed or is recessive. In other words we have a tall plant or a brown-eyed child.

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