OCR Rendition - approximate

290 Mr. Francis Galton [Feb. 9, admissible hypothesis that any two or more of them, such as reversion and natural selection, should follow laws so exactly inverse to one another that the one should reform what the other had deformed ; because characteristics, in which the relative importance of the various processes is very different, are none the less capable of conforming closely to the typical condition. When the idea first occurred to me, it became evident that the problem might be solved by the aid of a very mofterate amount of experiment. The properties of the law of deviation are not numerous, and they are very peculiar. All, therefore, thatt was needed from experiment was suggestion. I did not want proof, because the theoretical exigencies of the problem would afford that. What I wanted was to be started in the right direction. I will now allude to my experiments. I cast about for some time to find a population possessed of some measurable characteristic that conformed fairly well to the law, and that was suitable for investigation. I determined to take seeds and their weights, and after many preparatory inquiries, fixed upon those of sweet-peas. They were particularly well suited to my purposes; they do not cross-fertilise, which is a very exceptional condition ; they are hardy, prolific, of a convenient size to handle, and their weight does not alter when the air is damp or dry. The little pea at the end of the pod, so cha-. racteristic of ordinary peas, is absent in sweet-peas. - I weighed seeds individually, by thousands, and treated them as a census officer would treat a large population. Then I selected with great pains several sets for planting. Each set contained seven little packets, and in each packet were ten seeds, precisely of the same weight. Number one of the packets contained giant seeds, all as nearly as might be of + 3° of deviation. Number seven contained very small seeds, all of - 3° of deviation. The intermediate packets corresponded severally to the intermediate degrees ± 2°. ± 1° and 0°. As the seeds are too small to exhibit, I have cut out discs of paper in strict proportion to their sizes, and strips in strict proportion to their weights, and have hung below them the foliage produced by one complete set. Many friends and acquaintances each undertook the planting and culture of a complete set, so that I had simultaneous experiments going on in various parts of the United Kingdom. Two proved failures, but the final result was this : that I obtained the more or less complete produce of seven sets, that is, of 7 X 7 x 10, or 490 carefully weighed seeds. It would be wholly out of place if I were to enter into the details of the experiments themselves, the numerous little difficulties and imperfections in them, or how I balanced doubtful cases, how I divided returns into groups, to see if they confirmed one another, or how I conducted any other of the well-known statistical operations. Suffice it to say that I took immense pains, which, if I had understood the general conditions of the problem as clearly as I do now, I should not perhaps have cared to bestow. The results were most satisfactory. They gave me two data, which were all that I required in order to 1877.) on Typical Laws of Heredity. 291 understand the simplest form of descent, and so I got at the heart of the problem at once. Simple descent means this. The parentage must be single, as in the case of the sweet-peas which are not cross-fertilised, and the rate of production and the incidence of natural selection must both be independent of the characteristic. The only processes concerned in simple descent that can affect the characteristics of a sample of a population are those of Family Variability and Reversion. It is well to define these words clearly. By family variability is meant the departure of the children of the same or similarly descended families, from the ideal mean type of all of them. Reversion is the tendency of that ideal mean filial type to depart from the parent type, " reverting " towards what may be roughly and perhaps fairly described as the average ancestral type. If family variability had been the only process in simple descent that affected the characteristics of a sample, the dispersion of the race from its mean ideal type would indefinitely increase with the number of the generations ; but reversion checks this increase, and brings it to a standstill, under conditions which will now be explained. On weighing and sorting large samples of the produce of each of the seven different classes of the peas, I found in every case the law of deviation to prevail, and in every case the value of 1° of deviation to be the same. I was certainly astonished to find the family variability of the produce of the little seeds to be equal to that of the big ones ; but so it was, and I thankfully accept the fact ; for if it had been otherwise, I cannot imagine, from theoretical considerations, how the typical problem could be solved. The next great fact was that reversion followed the simplest possible law; the proportion being constant between the deviation of the mean weight of the produce generally and the deviation of the parent seed, reckoning in every case from one standard point. In a typical case, that standard must be the mean of the race, otherwise the deviation would become unsymmetrical, and cease to conform to the law. I have adjusted an apparatus (Fig. 1) to exhibit the action of these two processes. We may consider them to act not simultaneously, but in succession, and it is purely a matter of convenience which of the two we suppose to act the first. I suppose first Reversion, then Family Variability. That is to say, I suppose the parent first to revert, and then to tend to breed his like. So there are three stages: (1) the population of parents, (2) that of reverted parents, (3) that of their offspring: or, what comes to the same thing, (1) the population of parents, (2) that of the mean produce of each parent, (3) that of their actual produce. In arranging the apparatus I have supposed the population to continue uniform in numbers. This is a matter of no theoretical concern, as the whole of this memoir relates to the distinguishing peculiarities of samples irrespectively of the absolute number of individuals in those samples. The apparatus consists of a row of vertical compartments, with trap-doors below them, to hold pellets CIibPDF - www.fastio.com