MENTAL DEFICIENCY

Evening Post, Volume CVI, Issue 53, 11 September 1928, Page 11

MENTAL DEFICIENCY

MYSTERY OF MUTATIONS HEREDITY & HYBRIDISATION "WHAT WE CANNOT DO" By P. Fraser, M.P. 111. "What else do wo know of heredity besides the Mendelian laws? Professor J. Arthur Thompson, writing on "Heredity and Disease" in his work already mentioned, "Heredity," says that it must be admitted that ignorance on the whole subject is immense. This fact is borne out particularly in regard to what wo know as "Mutations.' In a field in Holland Hugo Do Vrics discovered a quantity of evening primroses growing, and he watched them carefully for several years. In the second year he found two perfectly distinct and new forms of plants, which he transplanted in his own garden and propagated, sowing their self-fertilised seed. When the seeds came up the new plants were found to breed true. Darwin and all the other biologists had believed that variations were minute, and that new forms came of small variations accumulated through soveyal generations. Here was evidenco that a now species might jump into cxistenco at once. Professor T. H. Morgan by experiments with tho fruit fly, drosophila, has .been able to establish that mutations are a common element in the appearance of new species. Tho late Luther Burbank made very practical use of "mutations" in tho production of now typeß of plants and fruits. It is now believed that probably the greatest advances in the discovery of new types of wheat and other domestic plants have, been due to careful search for sudden mutations rather than by eaieful breeding from small valuable variations. Nobody: knows whether or not there are mutations in human beings. Professor H. M. Newman says:— "In bringing his discussion of the causes of heritablo variations (mutations) to a closo, wo find ourselves in a somewhat pessimistic frame of mind. When all is said, it is found that our knowledge of what actually causes mutations is almost nothing. We think wo know something about the mechanism of heredity, but wo do not know tho mechanism of variation. The really great evolutionary discovery of the future will probably be the finding out of the cause or causes of mutations." In this connection Professor W. E. Castle, of Harvard, considers that "It seems to be a fatal . objection to the mutation theory as a general theory of evolution that the occurrence is not general." Professor Lotsky claims that he has proved that hybridisation does cause variation. He asserts that no two individuals that result from sexual union are exactly alike. He therefore, I undorstand, denies the existence of species, and regards every sexual mating as a hybridisation and sexual production as the cause of variation. Our own Dr. L. Cockayne (described by Dr. Hill, of Kew' Gardens, as one of the world's greatest botanists) and Esmond Atkinson, in their work "On the New Zealand Wild Hybrids of Nothofagus" state:— "We now come to a very critical part of our investigation, and one whjch demands the most searching investigation, since it has a distinct bearing on the question of evolution. A few hybrids camo to light, evidently xN. diffuse, with certain characters which we have not seen in either of the parents, nor can we bolieve that they are tho result of combining any of the ordinary visible parental characters. How they have come about we do not know, the explanation is for geneticists, our business alone is to present the facts such as they are." In all the available proved facts concerning tho Mendelian laws, mutations, and hybridisation there is, as yot, comparatively little of importance to the sociologist. CAN TEE CHROMOSOMES BE CONTROLLED? Tho situation is still further complicated by the Chromosomes. They seem quite capable of defying any legislative control. Every cell of every living being contains a constant and definite number of particles called chromosomes because, they are roadily stainabla and can.be easily seen when they have been made to absorb colouring. In many cases it has been found possible to count them, and it has been found that (with a few inexplicable exceptions) tho number is constant for each species. In some of the sharks the number is 3G; in the mouse, the salamander, tho trout, tho lily, 24; in the grasshopper it is 12; while in tho crustacean artemia the number is 168. • Although the number is said to be normally constant tho calculation of that number does not always seem to be, not at least as fat as man is coiit corned. I find that prior to. 1908 the number of .chromosomes in tho cells of tho ox, tho guinea pjg r and man, was estimated at 16, and tho onion was the proud possessor of the same number. About 1908 tho estimate for the ox, guinea pig, and man, was increased to 24, but I am afraid the onion was left stranded at the starting post with just the humble sixteen, and, probably with injured feeling at this treatment. Now, the ox, guinea pig, and man, are said to rejoice in the possession of 48 chromosomes in their colls, and certain kinds of snails have been elevated to take the placo of tho rojected onion alongside of man. In every living being half the chromosomes in each cell come from the father and half from tho l otlior by an elaborate process of splitting and growth. Prior to fertilisation the number of chromosomes in tho eggcell and the sperm cell are reduced by half during a series of changes so that the new being beginß life with the normal number of chromosomes. The importance of these chromosomes is that the biologists aay they can bo shown to be tho physical means whereby the Mendelian laws are carried out. Thoy aro the bearers of the "unit characters," the actual existence of which Professor Jennings seriously doubts, and their behaviour explains the operation of Mendel's laws. Dr. H. H. Goddard, in his book on "Feeble-mindedness: Its Causes and Consequences," states that it is now doubtful whether thoy are the sole bearers of "unit characters," while, as wo bavo already seen, Professor Jennings questions the conception of '' unit-characters'' altogether. Prol'«ssor Castle, of Harvard, in his "Goni\*'/cs and Eugenics," says: "In reality the chromosomes make up a part only of tho germ cell, and we have no certain knowledge that they form the most important part." Professor H. H. Zcight, taking tho number of chromosomes in man at 24 (they aro now estimated at 48) shows that there is always a theoretical possi-

bility in a human family of 109 difforont combinations of graudparental contributions. KNOWLEDGE OF HEREDITY. Professor Jennings in "Prometheus" multiplies tho possibilities many times in the following word 3 "Students of heredity, like other men, aro disposed to make the most of their achievements: to dwell upon what they know, what they can do, what they can predict. They have, indeed, achieved much; tho laßt twenty-five years have made greater advance in the knowledge of heredity than had all the ages before. But recognition of limitations is as valuable as other sorts ot knowledge; realisation (" what wo cannot do is as necei.sary for correct guidance as realisation of what we can do. Every pair of human parents contains thousands of pairs of the packets of chemicals on which development, depends. From these a set is drawn almost at random (subject to tho condition that one packet is taken from each pair possessed by each parent); this constitutes tho heritage of the child. Any pair of parents may thus produce, not merely thousands, but millions of different combinations, each yielding a child of different char acteristics. There is no way of controlling the combinations that shall enter into a child of given parents; there is no prospect that there ever will be. It is therefore, impossible to predict what kind of offspring will be produced by a given pair of parents—save in a fow respects, iii cases where the constitution of both packets of a particular pair are known for each parent. If both parents have tho corresponding pairs defective in the same manner — lacking, for example, something required for pi-oducing a normal mmd — then their children will be all defective like the parents; feeble-minded parents will produce feeble-minded children. But if, as may well bo the case, tho feeble-minded is due to defeeta in different packets in tho two parents, then all experimental breeding shows that the two parental stocks may supplement one another, so that the defect will not appear in thr offspring. The characteristics that are predictable are extremely few; a new combination is produced with every child. No pairof parents has a sure thing as to-the high character of their prospective offspring. There aro few parents, at least among those not falling in tho definitely pathological group, of whom it can be prodicted that their children must be worthless." [In tho next article we will consider how far the facts and theories of biology explain mental deficiency and other weaknesses in man.]