and Nearctic regions ought to be separated; and I determined if I made another region that it should be Madagascar.  I have therefore been able to appreciate the value of your evidence on these points.  What progress Palaeontology has made during the last 20 years!  But if it advances at the same rate in the future, our views on the migration and birthplace of the various groups will, I fear, be greatly altered.  I cannot feel quite easy about the Glacial period and the extinction of large mammals, but I much hope that you are right.  I think you will have to modify your belief about the difficulty of dispersal of land molluscs; I was interrupted when beginning to experimentise on the just-hatched young adhering to the feet of ground-roosting birds.  I differ on one other point, viz. in the belief that there must have existed a Tertiary Antarctic continent, from which various forms radiated to the southern extremities of our present continents.  But I could go on scribbling for ever.  You have written, as I believe, a grand and memorable work, which will last for years as the foundation for all future treatises on Geographical Distribution,—­My dear Wallace, yours very sincerely,

CHARLES DARWIN.

P.S.—­You have paid me the highest conceivable compliment by what you say of your work in relation to my chapters on Distribution in the “Origin,” and I heartily thank you for it.

* * * * *

The Dell, Grays, Essex.  June 7, 1876.

Dear Darwin,—­Many thanks for your very kind letter.  So few people will read my book at all regularly, that a criticism from one who does so will be very welcome.

If, as I suppose, it is only to p. 184 of Vol.  I. that you have read, you cannot yet quite see my conclusions on the points you refer to (land molluscs and Antarctic continent).  My own conclusions fluctuated during the progress of the book, and I have, I know, occasionally used expressions (the relics of earlier ideas) which are not quite consistent with what I say further on.  I am positively against any Southern continent as uniting South America with Australia or New Zealand, as you will see at Vol.  I., pp. 398-403 and 459-466.  My general conclusions as to Distribution of Land Mollusca[101] are at Vol.  II., pp. 522-529.  When you have read these passages and looked at the general facts which lead to them, I shall be glad to hear if you still differ from me.

Though, of course, present results as to origin and migrations of genera of mammals will have to be modified owing to new discoveries, I cannot help thinking that much will remain unaffected, because in all geographical and geological discoveries the great outlines are soon reached; the details alone remain to be modified.  I also think much of the geological evidence is now so accordant with, and explanatory of, geographical distribution that it is prima facie correct in outline.  Nevertheless, such vast masses of new facts will come out in the next few years that I quite dread the labour of incorporating them in a new edition.