irable; but your other metaphor, in which you compare classification
and complex machines, does not seem to me quite appropriate, though
I cannot point out what seems deficient. The point which seems to
me strong is that all naturalists admit that there is a natural
classification, and it is this which descent explains. I wish you had
insisted a little more against the "North British" (203/5. At page 485
Mr. Wallace deals with Fleeming Jenkin's review in the "North British
Review," 1867. The review strives to show that there are strict limits
to variation, since the most rigorous and long-continued selection
does not indefinitely increase such a quality as the fleetness of a
racehorse. On this Mr. Wallace remarks that "this argument fails to meet
the real question," which is, not whether indefinite change is possible,
"but whether such differences as do occur in nature could have been
produced by the accumulation of variations by selection.") on the
reviewer assuming that each variation which appears is a strongly marked
one; though by implication you have made this very plain. Nothing in
your whole article has struck me more than your view with respect to the
limit of fleetness in the racehorse and other such cases: I shall try
and quote you on this head in the proof of my concluding chapter. I
quite missed this explanation, though in the case of wheat I hit upon
something analogous. I am glad you praise the Duke's book, for I was
much struck with it. The part about flight seemed to me at first very
good; but as the wing is articulated by a ball-and-socket joint, I
suspect the Duke would find it very difficult to give any reason against
the belief that the wing strikes the air more or less obliquely. I have
been very glad to see your article and the drawing of the butterfly
in "Science Gossip." By the way, I cannot but think that you push
protection too far in some cases, as with the stripes on the tiger. I
have also this morning read an excellent abstract in the "Gardeners'
Chronicle" of your paper on nests. (203/6. An abstract of a paper on
"Birds' Nests and Plumage," read before the British Association: see
"Gard. Chron." 1867, page 1047.) I was not by any means fully converted
by your letter, but I think now I am so; and I hope it will be published
somewhere in extenso. It strikes me as a capital generalisation, and
appears to me even more original than it did at first...

I have finished Volume I. of my book ["Vari