I enclose two diagrams showing the sort of manner I CONJECTURE that mammals have been developed. I thought a little on this when writing page 429, beginning, "Mr. Waterhouse." (Please read the paragraph.) I have not knowledge enough to choose between these two diagrams. If the brain of Marsupials in embryo closely resembles that of Placentals, I should strongly prefer No.2, and this agrees with the antiquity of Microlestes.
As a general rule I should prefer No.1 diagram; whether or not Marsupials have gone on being developed, or rising in rank, from a very early period would depend on circumstances too complex for even a conjecture. Lingula has not risen since the Silurian epoch, whereas other molluscs may have risen.
Here appear two diagrams.
Diagram I.
A-Mammals, not true Marsupials nor true Placentals.
-2 branches - Branch I, True Placental, from which branch off Rodents, Insectivora, a branch terminating in Ruminants and Pachyderms, Canidae and terminates in Quadrumana.
-Branch II, True Marsupial, from which branches off Kangaroo family an unnamed branch terminating in 2 unnamed branches and terminates in Didelphys Family.
Diagram II.
A-True Marsupials, lowly developed.
-True Marsupials, highly developed.
-2 branches - Branch I, Placentals, from which branch off Rodents, Insectivora, a branch terminating in Ruminants and Pachyderms, Canidae and terminates in Quadrumana.
-Branch II, Present Marsupials, splitting into two branches terminating in Kangaroo family (with 2 unnamed branches) and Didelphys family.
A, in the two diagrams, represents an unknown form, probably intermediate between Mammals, Reptiles, and Birds, as intermediate as Lepidosiren now is between Fish and Batrachians. This unknown form is probably more closely related to Ornithorhynchus than to any other known form.
I do not think that the multiple origin of dogs goes against the single origin of man...All the races of man are so infinitely closer together than to any ape, that (as in the case of descent of all mammals from one progenitor), I should look at all races of men as having certainly descended from one parent. I should look at it as probable that the races of men were less numerous and less divergent formerly than now, unless, indeed, some lower and more aberrant race even than the Hottentot has become extinct. Supposing, as I do for one believe, that our dogs have descended from two or three wolves, jackals, etc., yet these have, on OURVIEW, descended from a single remote unknown progenitor. With domestic dogs the question is simply whether the whole amount of difference has been produced since man domesticated a single species; or whether part of the difference arises in the state of nature. Agassiz and Co. think the negro and Caucasian are now distinct species, and it is a mere vain discussion whether, when they were rather less distinct, they would, on this standard of specific value, deserve to be called species.
I agree with your answer which you give to yourself on this point; and the simile of man now keeping down any new man which might be developed, strikes me as good and new. The white man is "improving off the face of the earth" even races nearly his equals. With respect to islands, I think I would trust to want of time alone, and not to bats and Rodents.
N.B.--I know of no rodents on oceanic islands (except my Galapagos mouse, which MAY have been introduced by man) keeping down the development of other classes. Still MUCH more weight I should attribute to there being now, neither in islands nor elsewhere, [any] known animals of a grade of organisation intermediate between mammals, fish, reptiles, etc., whence a new mammal could be developed. If every vertebrate were destroyed throughout the world, except our NOW WELL-ESTABLISHED reptiles, millions of ages might elapse before reptiles could become highly developed on a scale equal to mammals; and, on the principle of inheritance, they would make some quite NEW CLASS, and not mammals; though POSSIBLY more intellectual!
I have not an idea that you will care for this letter, so speculative.
Most truly yours, C. DARWIN.
CHARLES DARWIN TO ASA GRAY.
Down, September 26 [1860].
...I have had a letter of fourteen folio pages from Harvey against my book, with some ingenious and new remarks; but it is an extraordinary fact that he does not understand at all what I mean by Natural Selection. I have begged him to read the Dialogue in next 'Silliman,' as you never touch the subject without making it clearer. I look at it as even more extraordinary that you never say a word or use an epithet which does not express fully my meaning. Now Lyell, Hooker, and others, who perfectly understand my book, yet sometimes use expressions to which I demur. Well, your extraordinary labour is over; if there is any fair amount of truth in my view, I am well assured that your great labour has not been thrown away...
I yet hope and almost believe, that the time will come when you will go further, in believing a very large amount of modification of species, than you did at first or do now. Can you tell me whether you believe further or more firmly than you did at first? I should really like to know this. Ican perceive in my immense correspondence with Lyell, who objected to much at first, that he has, perhaps unconsciousnessly to himself, converted himself very much during the last six months, and I think this is the case even with Hooker. This fact gives me far more confidence than any other fact.
CHARLES DARWIN TO C. LYELL.
15 Marine Parade, Eastbourne, Friday evening [September 28th, 1860].