WCP2091

[1]

London, 73 Harley S^t.

July 3 1867

My dear Mr Wallace

I was very glad [al]tho[ugh]' I take in the Westminster Review to have a duplicate of your most entertaining & instructive essay on mimicry of colours etc.[,]¹ which I have been reading with great delight & I may say that both copies are in full use here. I think it is admirably written & most persuasive.

I write to ask you whether you can help me to any theory of the occurrence of placental rodents in Australia. You have probably [2] speculated on their presence. Placental bats we can explain as Darwin does by their powers of flight & habits of trans-oceanic migration.²

Pritchard³ long ago alluded to the exceptional cases of rodents in oceanic islands accounting for it by saying that they may have been carried in canoes by natives;⁴ & now that we have enlarged our ideas of the antiquity of man & can carry him back to the Paleolithic period we may use canoes as a means of transport far back in time.

I do not know that in the rich extinct [3] marsupial fauna of the Wellington & other caves in Australia or in the alluvium of that continent there have yet been found any rodents contemporary with the Diprotodon & others. Is it conceivable that the placental rodents got into Australia after the mammalia of the marsupial type had been long established & dominant, that type having been adapted to perform the function & in a great measure to imitate the structure of the placental rodentia, as the wombat & others[.]

We can understand the absence of mammalia [4] of the higher grade in Australia just as we can explain the want of both of didelphous & monodelphous quadrupeds in New Zealand, namely by imagining that no representative of such types were able to reach such regions. But if placental rodents got to Australia as soon as any marsupials, ought they not to have been developed as much as the latter.

I am afraid we have not geological knowledge enough to reason safely on this subject. It is to me very satisfactory that in digging up the bones of extinct species of Dinosaur in New Zealand [5] [p.2] & the skeletons of the Dodo in the Mauritius no bones of fossil mammalia are found such as we meet with in the mammoth-bearing gravel of the Thames valley. It seems to show that the absence of mammalia dates far back in such cases & cannot be explained away by supposing that the earliest men extirpated them.

With many thanks for your essay on mimicry.

believe me | ever most truly yours | Cha Lyell [signature]

A. Wallace Esq