THE SPREAD OF MYXOMATOSIS

333 tortoises. These animals are very unlikely to contaminate food, so that the chances of their being the source of an outbreak of food-poisoning are remote. Nevertheless, they may be a risk to their individual owners and particularly to children. FAMILY SIZE AND INTELLIGENCE PERHAPS it was not ever thus ; but nowadays (as Mr. John D. Nisbet, PH.D., reminds us in a new occasional paper 1) dull children tend to be found in large families and intelligent children in small ones. Since large families are now commonest among the dullest of our - or anyhow the least successful-members society, and since their children may inherit their dullness, we are perhaps threatened with a decline in national intelligence. In 1946, Sir Godfrey Thomson2 and Sir Cyril Burt3 published studies suggesting that this decline had already begun, but later studies have failed to confirm their findings beyond question. Indeed the results have been paradoxical : for in the Scottish mental surveys of 1932 and 1947 the test-score was higher at the second survey. The matter is very difficult to investigate, because so many variables affect it. Thus the practice of having smaller families is steadily spreading through the social classes from above downwards ; and social class is to some extent related to intelligence. Mothers of low intelligence lose more children than those of better intelligence-though the effect of this differential deathrate must now be slight. Again, there is a tendency for the children of outstanding parents-whether outstandingly bright or outstandingly stupid-to regress towards the mean : they tend to differ from the average only about half as much as their exceptional parents. This is thought to be partly because exceptional people do not marry people as exceptional as themselves, and partly because they are exceptional to some extent for reasons which are not inherited-reasons to do with the environment. These various trends might all operate to preserve us from a decline in national intelligence. Indeed, if the least intelligent came in time to limit their families below replacement level it might be supposed that the average intelligence of the nation would rise. Nevertheless, whatever the future trends may be, it is important to find out now, if we can, why children from large families score lower in intelligence tests (especially in verbal group tests) than children from small families; for if it is due to differences in the environment - to the financial and educational advantages enjoyed by the small family-perhaps we could find social remedies; whereas if it is largely a matter of inheritance then it is a problem in eugenics. Dr. Nisbet proposes the hypothesis that words are more than tools of expression : they are symbols which greatly increase the efficiency of abstract thought. A child, then, whose opportunities for verbal development are limited will be likely to score less in a test-even in a test of general intelligence-than a child who has not suffered such limitation; and the child who belongs to a large family sees less of grown-up people than the child in a small family, and so has less chance of acquiring an adult vocabulary. If this is so, then part of the negative correlation of family size and intelligence may be explained by the fact that the family environment may be depressing a child’s test-score below the level of his inheritance. And indeed the intelligence of parents of large families may not be so dull as the score of their children would suggest. He has tested this hypothesis in three ways : (1) by partial correlation of family size and verbal ability, with 1. Family Environment.

Occasional Paper no. 8. London : Eugenics Society and Cassell. 1953, Pp. 51. 3s. 6d. 2. The Trend of National Intelligence. Occasional Paper no. 3. Eugenics Society, 1946. 3. Intelligence and Fertility. Occasional Paper no. 2. Eugenics Society, 1946. See also Lancet, 1947, i, 36, 222.

held constant ; (2) by correlation of family and size several tests with different verbal loadings ; and (3) by correlation of family size and intelligence-test score at different ages (in order to note the cumulative effect of the environment). In two groups, each of some 2500 children aged 11 and over, his results supported the hypothesis that the environment of a large family hinders verbal development, that this hindrance affects general mental development, and that the effect was cumulative with the passage of time. It is clear to him, however, that the whole of the negative correlation between family size and intelligence cannot be explained in terms of this environmental influence. There is still

intelligence

a substantial negative correlation when environmental factors have been allowed for. He argues that a general reduction in the size of families may have resulted in an artificial " rise in the mean score which might be sufficient to mask a real decline in national intelligence due to genetical loss. "

THE SPREAD OF MYXOMATOSIS LAST vear we mentioned1 the efforts made to rid Australia of one of her greatest pests, the rabbit, by setting a thief to catch a thief. Myxoma virus is, in theory, an ideal instrument of biological warfare against the rabbit, for it is highly pathogenic for wild and domestic rabbits, but harmless for men and other animals. Many attempts have been made to start myxoma epizootics by introducing infected animals into areas densely populated with rabbits. The attempts were unsuccessful until 1950-51 when, after a slow start, an epizootic flared up simultaneously in the Murray Valley where the virus had been released, and in many other places, including one spot 400 miles from the nearest known point of infection. The results of the epizootic have been difficult to evaluate, but it was estimated that tens of millions of rabbits died and that thousands of farms benefited by an increase in pasture growth and stock-bearing capacity. The disease was transmitted mechanically by several different species of mosquito. To account for the sudden appearance of epizootic centres in places far from the point where virus was released, it was assumed that the disease was spread by mosquitoes carried on air currents. Now, Breretonputs forward a new hypothesis of the spread of myxomatosis. He inquired into the times at which the disease was first recognised in different areas and, since all country folk were intensely interested in the experiment, the figures are thought to be reliable. The disease appeared to " creep " at a steady rate of about three miles a day, independently of the caseprevalence in an area. Brereton concludes that the disease is spread from one place to another at a constant rate by the movement of rabbits, while insect vectors determine the seasonal outbreaks. Brereton’s theories will be cold comfort to Dr. ArmandDelille, who recently started a private epizootic of myxomatosis in an attempt to wipe out the rabbits within his walled estate in the department of Eure-etLoir. The four walls may have kept the rabbits from creeping out, but the mosquitoes got in, and now myxomatosis is raging in 29 of the French departments.3 The disease is attacking domestic as well as wild rabbits, and breeders and manufacturers of small-arms ammunition face a crisis. Dr. Armand-Delille, impressed by the preliminary results of his experiment, read a paper on the subject to the Academy of Agriculture, and now that the experiment has continued longer than he had intended, he faces prosecution by the State for opening this Pandora’s box. Ironically, the Pasteur Institute is trying to produce sufficient vaccine to protect domestic rabbits. 1.

Lancet, 1952, ii, 278.

2. Breroton, J. C. U. Nature, Lond. 1953, 172, 108. 3. Manchester Guardian. July 24, 1953.