Xxxii MEMOIR. 
forty perfectly distinct species of this genus inhabiting this region, and of 
these no less than eighteen are endemic, all of them so peculiarly restricted 
in their range that they are not found, nor any forms closely representing 
them, even at twelve degrees of latitude on either side of the equator. The 
result is plain, that there has always (at least through immense geological 
epochs) been an equatorial fauna rich in endemic species, and that extinction 
cannot have prevailed to any extent within a period of time so comparatively 
modern as the glacial epoch in geology.’’ * 
This new aspect of the matter deeply interested Darwin. In 
his chapter on “Geographical Distribution” in the Ovigzz of Species, 
he had explained the presence of north temperate forms, chiefly 
plants, in the highlands of the southern hemisphere, as due “to 
a lowering of temperature during glacial epochs, which allowed 
these forms to migrate across the intervening tropical lowlands.” 
But, as Mr. Wallace points out, any such change within the epoch 
of existing species is almost inconceivable, the wind-carriage, and, 
in far lesser degree, the bird-carriage, of seeds explaining the dis- 
tribution without assuming the agency of vast physical changes.T 
The following correspondence deals with this question :— 
“Down, BROMLEY, KENT, JZarch 26fh, 1861. 
(Have you received copy of new edition of Ovzgz7 ?) 
‘< DEAR SIR, 
‘‘T have read your papers with extreme interest, and I have carefully 
read every word of them. They seem to me to be far richer in facts on 
‘variation, and especially on the distribution of varieties and sub-species, 
than anything which I have read. Hereafter I shall re-read them, and hope 
in my future work to profit by them and make use of them. The amount of 
variation has much surprised me. The analogous variation of distinct species 
in the same regions strikes me as particularly curious. The greater variability 
of female sex is new to me. Your Guiana case seems in some degree 
analogous, as far as plants are concerned, with the modern plains of La 
Plata, which seem to have been colonised from the north; but the species 
have been hardly modified. I have been particularly struck with your 
remarks on the glacial period. You seem to me to have put the case with 
admirable clearness and with crushing force. I am quite staggered with the 
blow, and do not know what to think. Of late several facts have turned up, 
leading me to believe more firmly that the glacial period did affect the 
equatorial regions, but I can make no answer to your arguments, and am 
completely in a cleft stick. By an odd chance I had only a few days ago 
been discussing the subject, in relation to plants, with Dr. Hooker, who 
believes to a certain extent; but strongly urged the little apparent extinction 
in equatorial regions. I stated in a letter, some days ago, to him that the | 
* Trans Entom. Soc., vol. V., pp. 352, 353- 
t+ Cf. Darwinism, pp. 371-3- 
