My dear Darwin

It was truly kind of you, to write me with your own hand, a note of warning about Balbiani;¹ but I do not use his statements in any way, in my forthcoming memoir which is to be read next Tuesday at the Anthropological Soc^y.²

The general line of it, is this:— First, I start with the 4 postulates, in favor 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 & variety (existing somewhere)

3. That undeveloped germs do not perish; but multiply and are transmissable.

4. Organization wholy 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 transmissable (here the sample was over rich & drained the more fecund residue)— (3.) The fact of some diseases skipping 1 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 residue, which has not strength to break out in the next generation, but being husbanded in a latent form, there multiplies & recovers strength to break out in the next or in a succeeding generation)

Next, I go into the question of affinities & repulsions; which I put as necessarily numerous & many sided (while professing entire ignorance of their character) & I argue thence, a long period of restless unsettlement in the newly fertilized ovum accompanied, as we know it to be, with numerous segregations & 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 shews, 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, &c, as well as for 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 shew, that the deductions 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 act 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 progeny may begin to vary under changed conditions sooner than the parent, (as in the hair or fleece of the young of dogs & sheep, transported to the tropics)

As regards Brown-Sèquard’s guinea-pigs;— If I rightly understand & am informed of his experiment, it is open to fatal objection.³ The guinea pigs that were operated on, appear to have been kept separate from the rest. If so, we sh^d. 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, &c)   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 modication, 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 & 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 organized society, coming from a foreign country. Thus I account both for the fact, and for the great rarity & slowness of the inheritance of acquired modifications

In conclusion, I restate a former definition, that I gave of the character of the relationship between parent and child, which I make out to be, not like that which connects a parent nation and its colonists, but like that which connects the representative government of the parent nation with the representative government of the colonists; with the further supposition, that the gov^t. of the parent country is empowered to nominate a small proportion of the colonists.

I have now, so far as the limits of a letter admit, made a clean breast of my audacity in theoretically differing from Pangenesis:—⁴

(1) in supposing the sexual elements to be of as early an origin as any part of the body (it was the emphatic declarations of Balbiani on this point that chiefly attracted my interest)⁵ and that they are not formed by aggregation of germs floating loose & freely circulating in the system, and (2) in supposing the personal structure to be of very secondary importance in Heredity, being as I take it, a sample of that which is of primary importance, but not the thing itself.—

If I could help, even in accustoming people to the idea that the notion of Organic Germs is certainly that on which the true theory of Heredity must rest, and that the question now is upon details & not on first principles, I should be very happy

Ever y^rs. Francis Galton

Thanks for the letter on the Hindoo family, wh: I will keep & for the pamphlet on the wholesale execution of weakly people, which I return by book post.⁶