WCP6895

[1]

Waldron Edge, Duppas Hill, Croydon

August 10th, 1879

Dear Sir

I quite admit with you that the warm polar climates are more difficult to account for satisfactorily than the glacial epochs,— and I have, I think, sufficiently indicated my belief that in their case geographical changes have almost ^necessarily occurred. If a much greater quantity of heat than now even then carried towards the pole by ocean currents, and if there were much less ice owing to the absence of high land, and if the whole of the N. temperate zone were kept much warmer than now in winter by the nearness of the sun at that season, I do not think any reasonable amount of clear weather & consequent radiation would do much harm. It is not all our clear nights in winter [2] that are cold, and we often have spells of mild December weather with clear days & nights, showing that when warm air is brought to us radiation matters little. No doubt great destruction might be occasionally wrought in the Miocene flora by exceptionally cold winters, but all the individuals of a species would not be destroyed any more than they are with us, & the gaps would soon be filled up. If we can ever believe that the conditions were such that there were no icefields in the Arctic seas, then the whole of the heat brought in by ocean currents would go to keep up a permanent warmth in the ^{water and} ice; and I think it is pretty clear if there were no high land, & the [3] gulf stream were more powerful, there would be no icefields. I have hardly dwelt sufficiently on this point in my paper.

As to glacial deposits in what are now tropical countries, there is of course always a doubt about them. But if real they may have been formed at great elevations. Glacial deposits are now forming in the Himalayas in 29° N.Lat ^{at about 16,000 ft. elevation,} & if these subsided & were covered by marine deposits they might be preserved to future ages. We know there have been such subsidences; why should not some of them have brought down glacial formations from great elevations to below the sea-level?

No doubt the Atlantic & Pacific oceans have at different times been united at Panama & other parts of Central America (See my Geog. Dist. of Animals, Vol.II, p.57). This might diminish the [4] force of the gulf-stream, but would probably not abolish it unless the strait at Panama were very deep. If it were shallow it might produce little effect. But it is not improbable that during early Tertiary times the greater Antilles were united directly with S. America (l.cl[?] p.78) in which case the whole body of water which goes to form the gulf stream would ^flow more directly into the N. Atlantic with less loss of velocity and by evaporation, so that the actual heating power to the north Atlantic from this cause alone might be greater than now. We can never calculate on any one geographical change with considering what other changes may have affected it at the same period.

Thanks for references & papers, which I will look at when I have the opportunity. I have received a very kind letter from Croll informing me that both he and James Giekie are much pleased with my article.

Believe me | Yours faithfully | Alfred R. Wallace [signature]

Revd. O. Fisher