28 SCIENTIFIC WOKK, 1860-1865 



from America, and they were not overtaken and reinforced 

 by the migrants on the American shore. 



These conclusions then, drawn from much laborious com- 

 parison of species and tabulation of statistics, could only be 

 accounted for by admitting Darwin's hypothesis of the south- 

 ward migration of northern forms — an hypothesis begun by 

 Edward Forbes and extended by Darwin to transtropical 

 migration. Nevertheless, Hooker felt doubts as to the extent 

 of the world-wide cooling invoked by Darwin to account for this 

 transtropical migration ; and the amount of equatorial cooling 

 needed would, he considered, have killed off all the purely 

 tropical vegetation such as we know. 



The same Darwinian interest extended to his technical 

 work on Mann's Cameroon plants, so interesting as connecting 

 the Cape and Europe. This was to lead to a discussion of the 

 cold period * qwoad tropical African Mountains and Flora,' and 

 letters to Darwin are full of information as to what northern 

 plants were preserved in the cooler tracts of these tropical 

 mountains. 



I do not know what to think of Tropical plants during 

 the cold period [he writes on March 17]. As to their living 

 through it, it is an impossibility. I quite go along with you 

 in suggesting as many Tertiary or Secondary cold periods of 

 migration as you please. But that such an order as Diptero- 

 carpeae, whose species are all ultra-tropical, all trees, contain- 

 ing many diverse genera and species, should have survived 

 a cold period, or have been developed since, are equally pre- 

 posterous surmises in the present state of science. 



Darwin in return repeated the claim of the * Origin ' (ch. 

 xi.) for no more tropical cooling than Hooker himself had found 

 in the Himalayan zone where tropical and temperate flora 

 commingled, and confidently believed it would be found that 

 the ultra-tropical plants mentioned could adapt themselves to 

 this amount of cooling in conjunction with other changes in 

 physical conditions, such as moisture. 



Against this, however, he still held out, writing on 

 March 18 : 



