186 Life and Letters of Francis Galton
for diseases skipping a generation. This selecting out of germs will not occur in animals of pure breeds for their stirp contains only one or a very few varieties of each species of germ, so that the selection will contain all, and thus the offspring resemble their parents and one another.
"The more mongrel the breed, the greater is the variety of the offspring." (p. 336.)
To this principle, however, Galton adds a limitation, the stirp cannot be indefinitely increased in complexity, because there is a limit to the space it occupies. There is a finite, if great, number of varieties of germs, and of the individual germs in each variety.
"Thus in the gradual breeding out of negro blood, we may find the colour of a mulatto to be the half, and that of a quadroon to be the quarter of that of his black ancestors; but as we proceed further, the sub-division becomes very irregular; it does not continue indefinitely in
the geometrical series of one-eighth, one-sixteenth, and so on, but is usually present very obviously or not at all, until it entirely disappears." (p. 335.)
Turning now to the germ which has developed into a somatic cell, Galton questions whether it does produce gemmules at all-at any rate its fertility is far less than that of the latent germ. Influences acting on the somatic cells of the parent are only slightly or not at all represented in the like somatic cells of the offspring. He considers at some length instances of inherited mutilations and of acquired characters, and thinks they may be reasonably looked upon as a `collection of coincidences.' Even if there are real cases of changes in the somatic cells of the parents influencing the 'somatic characters of the offspring, Galton would but admit that occasionally gemmules are thrown off by somatic cells, which find their way into the circulation and ultimately obtain a lodgment in the already constituted sexual elements. Such a process is, however, independent of and subordinate to the causes which mainly govern heredity (pp. 347-88). Even to the last Galton did not wholly give up Pan-genesis, for Darwin had accepted Brown-Sequard's epileptic guinea-pigs, yet as Galton remarked
"It is indeed hard to find evidence of the power of the personal structure to react upon the sexual elements that is not open to serious objection." (p. 345.)
Finally I may cite:
"The hypothesis of organic units enables us to specify with much clearness the curiously circuitous relation which connects the offspring with its parents. The idea of its being one of direct descent, in the common acceptation of that vague phrase, is wholly untenable, and is the
chief cause why most persons seem perplexed at the appearance of capriciousness in hereditary transmission. The stirp of the child may be considered to have descended directly from a part of the stirps of each of its parents, but then the personal structure of the child is no more than an imperfect representation of his own stirp, and the personal structure of each of the parents is no more than an imperfect representation of each of their own stirps." (p. 3461.)
Such a modern idea as that parents are only conduit-pipes for the germplasm of their stocks is fully expressed by Galton with better limitation, and with fuller suggestiveness, both in this paper and in that on Blood
From the modern biometric standpoint the association is 'correlational' not causal.