Mechanism of Variation

New Zealand Herald, Volume LXXV, Issue 22955, 5 February 1938, Page 10 (Supplement)

Mechanism of Variation

Plant and Animal Evolution

Specially Written for the New Zealand Herald by R. A. FALLA, M.A.

HE idea that all existing plants I and animals havev been produced by natural causes from simpler original forms or from one primordial material is a very old one. FERVID FEELING The process is known as organic evolution, and it has loomed so largely in the scientific and philosophical thought of the last century that the name itself is now sufficient to arouse a good deal of fervid feeling on the part of those who consider that the study of evolutionary theories helps them to see the way to social progress, and equally fervid feeling on the part of a good number who think otherwise. MATTER OF TRAINING I venture to think that one's philosophy of life is often more a matter of training and temperament than the irresistibly logical outcome of theories tested and proved. It is difficult for every person to resist a tendency to become dogmatic, and even in the field of scientific inquiry it is sometimes forgotten that a working theory may be out of place as a gospel. The history of theories of organic evolution is concerned with a search for the means whereby it has been effected, and the

present position is that faith in a single primary cause has given place to the recognition of several important factors, and a deal of argument as to which is the most important of them. I.AMARCK'S EXPLANATION For many years the explanation offered by Lamarck was considered satisfactory. He held, that persistent efforts in a certain direction by animals induced variation; * and that slight changes of form thus induced during tho life of the individual are transmitted to its offspring. It was supposed, ms the hackneyed example has it, that the giraffe as a species acquired its long .neck by persistent effort to, reach higher and higher leaves. In it& simple form this theory is no., longer held. It received what should have been a deathblow when Weismavln showed experimentally that acquired characteristics appear never to be. inherited. But whenever new discoveries put all the theories in the melting pot Lamarckism usually emerges in a fresh guise. A geologist told me recently that he thought many biologists were inclined to give lipservice to the theories associated with Darwin and to act as if they believed that Lamarck was right. NATURAL SELECTION It was Darwin, simultaneously with Wallace, who proposed the explanation that natural selection was the determining factor in organic evolution. 1 his hypothesis began with the facts of variation, heredity, and the tendency of animals to multiply. Domestication and the controlled breeding of animals over many years had already shown that certain variations of form or colour could be singled out and perpetuated by selective breeding. Darwin was able to

demonstrate that in nature a selective process also operates, hence the term natural selection. According to this the hypothetical explanation of, for example, the whiteness of polar bears is as follows. At some stage bears both brown and white inhabited the Arctic regions, but accompanying the white fur were certain advantages of greater warmth and protective concealment so that more and more white bears tended to survive until they replaced the brown in those regions. MENDEL'S EXPERIMENTS The Darwinian position must now be defended on different grounds from those on which it originated. Firstly the study of heredity based on Mendel s experiments, and secondly the recent study of the nature of variation have shown that the minute chance variations which Darwin supposed might be the " raw material " upon, which natural selection operated seem to ha\ e no survival value. On the other hand the occurrence of discontinuous variations or mutations provides something on which natural selection can operate. The exact nature of variations and mutations remained a matter for speculation until the rise of the modern science of genetics. CONSTITUENT CELLS

This study carries investigation beyond externals and structural anatomy to the constituent cells of the organism. For some species of aiiinicils unci rn«inj

of plants it is now possible to say that their cells contain so many chromosomes, and this number normally remains fixed for the species. It is found also that a mutation may be such a deep-seated change as to involve the number of chromosomes, and the experimental geneticist can use fiucli mutants to establish new varieties, it is even claimed that, under rare conditions where two mutants produce fertile hybrids the geneticist can establish what most biologists would call a species. In a practical way this work is being applied in plant breeding in many places including the New Zealand Wheat Research Institute. PURE CHANCE FACTOR The -causes of inherited variation are still beyond the range of . our knowledge. New discoveries in physical science complicate tlie problem; variation can be induced by X-rays, and appears to be afiected also, by cosmic rays, so that there is little excuse for scientific dogmatism at tho present stage of investigation. "'The real life of animals," writes Charles Elton in a recent book, " is a compound of many things: fixed and pre-determined limits impressed by the environment; the relations of the sexes; the survival of things that are useful; a certain free will in tho matter of choosing between good and evil surroundings, accompanied by a great deal of movement; a fairly iarge amount of pure chance; and sometimes a growing stock of new ideas born out of contact with new situations Predetermination, Sex, Materialism, Free Will, Destiny, Originality, and Tradition." Such an analysis is another way of saying that Truth can be approached from mc a than one angle.