I am sorry that you had the trouble of returning the Dog MS., which I suppose I shall receive to-morrow.
I mean to give good woodcuts of all the chief races of pigeons. (112/2. "The Variation of Animals and Plants under Domestication," 1868.)
Except the C. oenas (112/3. The Columba oenas of Europe roosts on trees and builds its nest in holes, either in trees or the ground ("Var. of Animals," Volume I., page 183).) (which is partly, indeed almost entirely, a wood pigeon), there is no other rock pigeon with which our domestic pigeon would cross--that is, if several exceedingly close geographical races of C. livia, which hardly any ornithologist looks at as true species, be all grouped under C. livia. (112/4. Columba livia, the Rock-pigeon. "We may conclude with confidence that all the domestic races, notwithstanding their great amount of difference, are descended from the Columba livia, including under this name certain wild races" (op. cit., Volume I., page 223).)
I am writing higgledy-piggledy, as I re-read your letter. I thought that my letter had been much wilder than yours. I quite feel the comfort of writing when one may "alter one's speculations the day after." It is beyond my knowledge to weigh ranks of birds and monotremes; in the respiratory and circulatory system and muscular energy I believe birds are ahead of all mammals.
I knew that you must have known about New Guinea; but in writing to you I never make myself civil!
After treating some half-dozen or dozen domestic animals in the same manner as I treat dogs, I intended to have a chapter of conclusions. But Heaven knows when I shall finish: I get on very slowly. You would be surprised how long it took me to pick out what seemed useful about dogs out of multitudes of details.
I see the force of your remark about more isolated races of man in old times, and therefore more in number. It seems to me difficult to weigh probabilities. Perhaps so, if you refer to very slight differences in the races: to make great differences much time would be required, and then, even at the earliest period I should have expected one race to have spread, conquered, and exterminated the others.
With respect to Falconer's series of Elephants (112/5. In 1837 Dr. Falconer and Sir Proby Cautley collected a large number of fossil remains from the Siwalik Hills. Falconer and Cautley, "Fauna Antiqua Sivalensis," 1845-49.), I think the case could be answered better than I have done in the "Origin," page 334. (112/6. "Origin of Species," Edition I., page 334. "It is no real objection to the truth of the statement that the fauna of each period as a whole is nearly intermediate in character between the preceding and succeeding faunas, that certain genera offer exceptions to the rule. For instance, mastodons and elephants, when arranged by Dr. Falconer in two series, first according to their mutual affinities and then according to their periods of existence, do not accord in arrangement. The species extreme in character are not the oldest, or the most recent; nor are those which are intermediate in character intermediate in age. But supposing for an instant, in this and other such cases, that the record of the first appearance and disappearance of the species was perfect, we have no reason to believe that forms successively produced necessarily endure for corresponding lengths of time. A very ancient form might occasionally last much longer than a form elsewhere subsequently produced, especially in the case of terrestrial productions inhabiting separated districts" (pages 334-5). The same words occur in the later edition of the "Origin" (Edition VI., page 306.) All these new discoveries show how imperfect the discovered series is, which Falconer thought years ago was nearly perfect.
I will send to-day or to-morrow two articles by Asa Gray. The longer one (now not finally corrected) will come out in the October "Atlantic Monthly," and they can be got at Trubner's. Hearty thanks for all your kindness.