182 Life and Letters of Francis Galton
42, RUTLAND GATE, S. W. Nov. 3/75.
MY DEAR DARWIN, It was truly kind of you, to write me with your own hand, a note of warning about Balbiani; but I do not use his statements in any way, in my forthcoming memoir which is to be read next Tuesday at the Anthropological Society.
The general line of it is this
First I start with the 4 postulates, in favour of which you have so strongly argued, and which may reasonably be now taken for granted:
1. Organic units in great number.
2. Germs of such units in still greater number and variety (existing somewhere).
3. That undeveloped germs do not perish; but multiply and are transmissible.
4. Organisation wholly depends on mutual affinities.
From these 4 postulates, I logically deduce several results, one of which is the importance and almost the necessity of double parentage in all complex organisations, and consequently of sex.
Then I argue that we must not look upon those germs that achieve development as the main sources of fertility; on the contrary, considering the far greater number of germs in the latent state, the influence of the former, i.e. of the personal structure, is relatively insignificant. Nay further, it is comparatively sterile, as the germ once fairly developed is passive; while that which remains latent continues to multiply. From this follows:
(1) The extremely small transmissibility of acquired modifications (to which I recur).
(2) The fact that exceptional gifts are sometimes barely transmissible (here the sample
was over rich and drained the more fecund residue).
(3) The fact of some diseases skipping one or more generations; (here the supposition is
made of the germs of those diseases being peculiarly gregarious, hence the general
outbreak of them leaves but a small residuum which has not strength to break out in
the next generation, but being husbanded in a latent form, there multiplies and re
covers strength to break out in the next or in a succeeding generation).
Next, I go into the question of affinities and repulsions, which I put as necessarily numerous and many-sided (while professing entire ignorance of their character) and I argue thence, a long period of restless unsettlement in the newly fertilised ovum, accompanied as we know it to be, with numerous segregations and segmentations in each of which the dominant germs achieve development, while the residue is segregated to form the sexual elements. But I argue, that as our experience of political and other segregations shows that they are never perfect, we are justified in expecting that numerous alien germs will be lodged in every structure and that specimens of all of them will be found in almost all parts of the body. In this way, I account for the reproduction of lost parts, etc., as well as foil the inheritance of all peculiarities that had been congenital in an ancestor.
I then consider the cases of inheritance of what had been non-congenital in an ancestor, but acquired by him. I show that the deduction usually made, that the structure reacts on the sexual elements, is not justified by the evidence of adaptivity of race, when this depends on conditions which curt equally on all parts of the body. My reason is, that since the same agents (viz. the germs) are concerned both in growth and in reproduction, the conditions that would modify the one, would simultaneously modify the other; hence they would be collaterally affected and the apparent inheritance is not a case of inheritance at all, in the strict sense of the word. Nay the "progress may begin to vary under changed conditions sooner than the parent (as in the hair or fleece of the young of dogs and sheep, transported to the tropics).
As regards Brown-Sequard's guinea-pigs;;-if I rightly understand and am informed of his experiment, it is open to fatal objection. The guinea-pigs that were operated on appear to have been kept separate from the rest. If so, we should expect the young sometimes to have convulsive attacks from mere imitation, just as we should expect of children brought up in a ward of epileptic patients, or among hysterical people (revivals, dancing mania etc.). Besides, there is not the least evidence that the mutilation of the spinal marrow, on which the parental epilepsy primarily depended, was inherited. I also disparage much other evidence of the inheritance of acquired modifications, leaving but a very small residue to accept. For this residue, I account by supposing the germs thrown off by the structure during its regular reparation, to frequently find their way into the circulation and some of these occasionally to reach the sexual elements and to become lodged and naturalised there, either by finding an unoccupied place or by dislodging others, like immigrants into an organised society, coming from a foreign country. Thus I account both for the fact, and for the great rarity and slowness of the inheritance of acquired modifications.