But even 12,000 feet would not be enough for the Azores, or for Hooker's continent; I believe Hooker does not infer a continuous continent, but approximate groups of islands, with, if we may judge from existing continents, not PROFOUNDLY deep sea between them; but the argument from the volcanic nature of nearly every existing oceanic island tell against such supposed groups of islands,--for I presume he does not suppose a mere chain of volcanic islands belting the southern hemisphere.
FOURTHLY.
The supposed continental extensions do not seem to me, perfectly to account for all the phenomena of distribution on islands; as the absence of mammals and Batrachians; the absence of certain great groups of insects on Madeira, and of Acaciae and Banksias, etc., in New Zealand; the paucity of plants in some cases, etc. Not that those who believe in various accidental means of dispersal, can explain most of these cases; but they may at least say that these facts seem hardly compatible with former continuous land.
FINALLY.
For these several reasons, and especially considering it certain (in which you will agree) that we are extremely ignorant of means of dispersal, I cannot avoid thinking that Forbes' 'Atlantis,' was an ill-service to science, as checking a close study of means of dissemination. I shall be really grateful to hear, as briefly as you like, whether these arguments have any weight with you, putting yourself in the position of an honest judge. I told Hooker that I was going to write to you on this subject; and I should like him to read this; but whether he or you will think it worth time and postage remains to be proved.
Yours most truly, CHARLES DARWIN.
[On July 8th he wrote to Sir Charles Lyell.
"I am sorry you cannot give any verdict on Continental extensions; and I infer that you think my argument of not much weight against such extensions. I know I wish I could believe so."]
CHARLES DARWIN TO ASA GRAY. Down, July 20th [1856].
...It is not a little egotistical, but I should like to tell you (and I do not THINK I have) how I view my work. Nineteen years (!) ago it occurred to me that whilst otherwise employed on Natural History, I might perhaps do good if I noted any sort of facts bearing on the question of the origin of species, and this I have since been doing. Either species have been independently created, or they have descended from other species, like varieties from one species. I think it can be shown to be probable that man gets his most distinct varieties by preserving such as arise best worth keeping and destroying the others, but I should fill a quire if I were to go on. To be brief, I ASSUME that species arise like our domestic varieties with MUCH extinction; and then test this hypothesis by comparison with as many general and pretty well-established propositions as I can find made out,--in geographical distribution, geological history, affinities, etc., etc. And it seems to me that, SUPPOSING that such hypothesis were to explain such general propositions, we ought, in accordance with the common way of following all sciences, to admit it till some better hypothesis be found out. For to my mind to say that species were created so and so is no scientific explanation, only a reverent way of saying it is so and so. But it is nonsensical trying to show how I try to proceed in the compass of a note. But as an honest man, I must tell you that I have come to the heterodox conclusion that there are no such things as independently created species--that species are only strongly defined varieties. I know that this will make you despise me. I do not much underrate the many HUGE difficulties on this view, but yet it seems to me to explain too much, otherwise inexplicable, to be false. Just to allude to one point in your last note, viz., about species of the same genus GENERALLY having a common or continuous area; if they are actual lineal descendants of one species, this of course would be the case; and the sadly too many exceptions (for me) have to be explained by climatal and geological changes.