[Enclosure 1]

Dell of Abernethy | Grantown N. B.

20 July 74

Dear Sir,

Your letter reached me on the 14^th. as I was starting for Scotland, and I have since been scarcely in condition for the study which its elaborate argument requires. Moreover, having now for some years been occupied with other subjects, it takes some time to revive the appropriate ideas with adequate clearness.

Doubtless the conceptions you have set forth are important—some of them being, I think, well made out. The difference in the mode of action of use and disuse is manifest as soon as it is pointed out, and though, at first, I did not see what was meant by the cessation of selection as a separate cause, I think you are right in asserting that it is one.

You have made it manifest that the process of indirect equilibration is more complex than at first appears; but I think it is even more complex than you have shewn— so complex that I question whether even approximate quantitative determinations are possible. I will name some of the factors which add to the complication.

1. The size of the organ. If large economy of nutrition will affect it rapidly; if small not.

2. Its distance from the centres of nutrition. If remote from the heart it is likely to dwindle faster than if near.

3. Its relation to the nutritive canals &c, which for the re-establishment of equilibrium, require to be changed all the way back to the centre.

4. The character of its tissue, as cheap or costly to the system, and as requiring much or little replacement: muscle would probably dwindle much more rapidly than horn or other dermal appendage.

5. The degree in which the organ is not only a direct or indirect cost—i.e. as a weight to be carried, here at much mechanical disadvantage and here at little, and as an inconvenience, hindering other action.

Some of the lines of inquiry here indicated may perhaps be worth pursuing

I am faithfully your’s, | Herbert Spencer

[Enclosure 2]

Dunskaith | Parkhill | Ross-shire

July 22/74

Abstract of letter to M^r. Herbert Spencer.

You observe that “the process of indirect equilibration is more complex than at first appears; … —so complex that I question whether even approximate quantitative determinations are possible.” From this I gather that you are now inclined to doubt whether it is possible to make “even approximate quantitative determinations”, as to the comparative potency of disuse (i.e., a directly equilibrating cause), and that of other reducing causes; whether these are of a directly or of an indirectly equilibrating character.⁴ In other words, I gather that you now deem it impossible to determine, even approximately, what share disuse has had in the reducing of any given structure; altho, of course, we may perceive that disuse has presumably had a greater share in the reducing of some structures, than it has had in the reducing of others. This distinction is of great importance in its bearing upon my previous letter; for the question there dis-cussed has no reference to the varying degrees in which disuse may operate, but only to a comparison of its influence with that of other reducing causes. Disuse is not compared with itself in different cases, but with the aggregate of other reducing causes in all cases. This distinction clear, I may proceed to observe that it is by way of necessary consequence that I gather your opinion to be such as just mentioned. For any difficulty we may experience in trying to make a quantitative determination in either of the two categories of reducing causes, must be an exact measure of the difficulty we shall experience, if we endeavour to make a similar determination in the other category—the influences comprised within the two categories being complementary to one another.

The suggestions with which you close your letter are very valuable to me—not the less so because two out of the five express opinions wh. I held before. At the end of my letter to Nature on the “cessation of selection,” I observed that the influence of this cause, considered in relation to that of the economy of nutrition, probably increases as the size of the affected structure decreases;⁵ and my reason for supposing this, is that wh. you tersely express in the words, “If large economy of nutrition will affect the organ rapidly; if small not.” In connection with this point, however, I shd. like to draw your attention to the following sentence in my letter to Nature, headed “Natural Selection and Dysteleology”⁶—a letter wh. I did not enclose to you, because not bearing upon the subject of my previous communication:—“Without entering into details, I think there are very good reasons for supposing, that the economy of growth is not able to reduce an organ wh. was originally large, to the same absolute size as it can an organ wh. was originally small.” The “reasons” here will readiiy occur to you.

The other of your opinions wh. agrees with a previously formed one of mine, is that relating to “the character of the tissue, as cheap or costly to the system”, etc. I have always felt this to be a very important consideration, and it was uppermost in my mind when I wrote, “moreover, we shd. scarcely expect disuse alone to affect in so similar a degree, such widely different tissues as are brain and muscle”.⁷

I remain etc | Geo. J. Romanes.