60 Life and Letters of Francis Galton
p. 185), that is of the continuity of the germ-plasm. It is only in a figurative sense that we can look upon the inheritance of the individual as a mosaic, and speak of the contribution of an ancestor to the result. The individual is the product of the germ-plasms that go to his production, not of the individual ancestors. The study of the characters of the individual ancestors is only ancillary to a study of the possibilities of those germ-plasms. The correlation of a somatic character in a great grandparent, say, and great grandchild is not in any sense a real measure of what the former contributes to the latter, nor is the corresponding multiple regression coefficient such a measure. We are testing what on the average we can predict of the somatic characters of the offspring from a knowledge of what the germ-plasms of the " stirp " have produced in the past. In other words the term ' " contribution of an ancestor" should be interpreted as, or be replaced by, " contribution of the ancestor to the prediction formula." It is in no sense a physical contribution to the germ plasms on which the somatic characters of the offspring depend. I do not think that anyone acquainted with the theory of multiple correlation would interpret the Law of Ancestral Heredity in any other sense ; but Galton's use of the terms "particulate inheritance," "mosaic," "heritage from distant progenitors," must be admitted to be easily capable of misinterpretation.
Galton then deals with the "heritages that blend and those that are mutually exclusive," citing as an illustration of the former, skin-colour in crosses between white and negro, and of the latter eye-colour. He does not here, any more than in his fundamental paper on eye-colour (see our p. 34), explain for what reason he assumes the distribution of eye-colour in the array of offspring due to a definite ancestry will be in the same proportions as in the case of a blended character in an individual offspring. Galton concludes that
"There are probably no heritages that perfectly blend, or that absolutely exclude one another, but all heritages have a tendency in one or the other direction, and the tendency is often a very strong one.... A peculiar interest attaches itself to mutually exclusive heritages, owing to the aid they must afford to the establishment of incipient races." (pp. 13-14.)
So far, however, as the struggle for existence and evolution are concerned, this last sentence must mean that a mosaic of the characters of two distinct races is for some environmental reasons more fitting than either pure race, and what is more, that the characters in the new mixed race will be stable and not segregate out again.
In the concluding paragraph we read
"The incalculable number of petty accidents that concur to produce variability among brothers, makes it impossible to predict the exact qualities of any individual from hereditary data. But we may predict average results with great certainty, as will be seen further on, and we can also obtain precise information concerning the penumbra of uncertainty that attaches itself to single predictions. It would be premature to speak further of this at present; what has been said is enough to give a clue to the chief motive of this chapter. Its intention has been to show the large part that is always played by chance in the course of hereditary transmission, and to establish the importance of an intelligent use of the laws of chance and of the statistical methods that are based upon. them, in expressing the conditions under which heredity acts." (pp. 16-17.)