OCR Rendition - approximate

282 Mr. Francis Gallon [Feb. 9, WEEKLY EVENING MEETING, Friday, February 9, 1877. Sin W. FREDERiex POLLOCK, Bart. M.A. Vice-President, in the Chair. FRANCIS GALTON, Esq. F.R.S. F.G.S. M.R.I. Typical Laws of Heredity. WE are far too apt to regard common events as matters of course, and to accept many things as obvious truths which are not obvious truths at all, but present problems of much interest. The problem to which I am about to direct attention is one of these. Why is it, when we compare two groups of persons selected at random from the same race, but belonging to different generations of it, we find them to be closely alike? Such statistical differences as there may be, are always to be ascribed to differences in the general conditions of their lives ; with these I am not concerned at present ; but so far as regards the processes of heredity alone, the resemblance of consecutive generations is a fact common to all forms of life. In each generation there will be tall and short individuals, heavy and light, strong and weak, dark and pale ; yet the proportions of the innumerable grades in which these several characteristics occur tend to be constant. The records of geological history afford striking evidences of this statistical similarity. Fossil remains of plants and animals may be dug out of strata at such different levels, that thousands of generations must have intervened between the periods in which they lived; yet in large samples of such fossils we seek in vain for peculiarities that will distinguish one generation taken as a whole from another, the different sizes, marks, and variations of every kind, occurring with equal frequency in both. The processes of heredity are found to be so wonderfully balanced, and their equilibrium to be so stable, that they concur in maintaining a perfect statistical resemblance so long as the external conditions remain unaltered. If there be any who are inclined to say there is no wonder in the matter, because each individual tends to leave his like behind him, and therefore each generation must resemble the one preceding, I can assure them that they utterly misunderstand the case. Individuals do not equally tend to leave their like behind them, as will be seen best from an extreme illustration. Lot us then consider the family history of widely different groups, 1877.] on Typical Laws of Heredity. 283 pay of 100 men, the most gigantic of their race and time, and the same number of medium men. Giants marry much more rarely than medium men, and when they do marry they have but few children. It is a matter of history that the more remarkable giants have left no issue at all. Consequently the offspring of the 100 giants would be much fewer in number than those of the medium men. Again, these few would, on the average, be of lower stature than their fathers, for two reasons. First, their breed is almost sure to be diluted by marriage. Secondly, the progeny of all exceptional individuals tends to "revert" towards mediocrity. Consequently the children of the giant group would not only be very few, but they would also be comparatively short. Even of these the taller ones would be the least likely to live. It is by no means the tallest men who best survive hardships ; their circulation is apt to be languid and their constitution con sumptive. It is obvious from this that the 100 giants will not leave behind them their quota in the next generation. The 100 medium men, on the other hand, being more fertile, breeding more truly to their like, being better fitted to survive hardships, &c., will leave more than their proportionate share of progeny. This being so, it might be expected that there would be fewer giants and more medium-sized men in the second generation than in the first. Yet, as a matter of fact, the giants and medium-sized men will, in the second generation, be found in the same proportions as before. The question, then, is this : How is it, that although each individual does not as a rule leave his like behind him, yet successive generations resemble each other with great exactitude in all their general features? It has, I believe, become more generally known than formerly, that although the characteristics of height, weight, strength, and fleetness are very different in themselves, and though different species of plants and animals exhibit every kind of diversity, yet the differences in height, weight, and every other characteristic, among members of the same species, are universally distributed in fair conformity with a single law. The phenomena with which that law deals are like those perspectives spoken of by Shakespeare, which, when viewed awry, show nothing but confusion. Our ordinary way of looking at individual differences is awry : thus we naturally, but wrongly, judge of differences in stature by differences in heights measured from the ground, whereas on changing our point of view to that whence the law of deviation regards them, by taking the average height of the race, and not the ground, as the point of reference, all confusion disappears, and uniformity prevails. It was to Quetelet that we were first indebted for a knowledge of the fact, that the amount and frequency of deviation from the average among members of the same race, in respect to each and every characteristic, tends to conform to the mathematical law of deviation. The diagram contains extracts from some of the tables by which CIibPDF - www.fastio.com