Sir Joseph Dalton Hooker. 



xv 



circumstances favoured ? " On this Darwin commented : " I rather demur 

 to your doctrine of centrifugal variation" (M.L., vol. 1, p. 199), and seemed to 

 think that it conflicted with his " Doctrine of Diversification." This is the 

 principle already mentioned, that the greatest amount of life can be 

 supported by great diversification of structure (' Origin,' 6th Ed., p. 88). 

 Hooker's own observations in the Khasia had taught him this. His own 

 point was different, and the explanation he gives, " the best marked varieties 

 of a wild species occurring on the confines of the area the species inhabits," 

 is undoubtedly the fact that proves the point. Both Hooker and Darwin 

 rejected the popular belief that species which had varied could revert to the 

 original type. In other words, evolution is not a reversible process, as 

 Hooker clearly saw. F. Darwin in discussing the point observes that this 

 does not conflict with Galton's " Eegression to Mediocrity," which is a 

 " centripetal tendency." For that only applies to a population which inter- 

 breeds freely, where the amount of variation always regresses to what Galton 

 terms the median value. Such regression tends to wipe out variability and 

 to establish racial stability. But the median is itself subject to variation 

 and to natural selection. 



The essay on the Australian flora has so far been dealt with only as an 

 apologia for Hooker's position with regard to Darwinian theory. As was 

 rightly stated on the occasion of the award to him of the Copley Medal, " it 

 effected a revolution " in respect to the rational basis on which he placed 

 geographical botany. De Candolle in his monumental work, 1 Geographie 

 Botanique Baisonnee,' published in 1855, only four years before the ' Origin,' 

 left the problem unsolved. Asa Gray remarks truly, " De Candolle's great 

 work closed one epoch in the history of the subject, and Hooker's name 

 is the first that appears in the ensuing one." 



In 1881 Hooker made Geographical Distribution the subject of his address 

 as President of the Geographical Section at the Jubilee meeting of the 

 British Association at York. He showed that from Linnseus onwards the 

 distribution of plants was regarded as dependent solely on physical conditions. 

 Meyen, for example, in his ' Geography of Plants ' (1836), of which the 

 Bay Society published a translation in 1846, lays down that " conditions of 

 climate, particularly heat and moisture, are the chief causes which determine 

 the station and distribution of plants" (p. 8). Hooker in this address 

 pointed out that such conclusions failed to give any explanation of the 

 occurrence of similar organisms "when there is no discoverable similarity 

 of physical conditions, and of their not occurring in places where the 

 conditions are similar " (p. 7). Dependence on physical features was still, 

 however, maintained by Grisebach in 1872. 



In the concluding pages of the essay on the Australian flora Hooker 

 briefly states the general conclusions at which he had arrived as to the 

 actual facts of plant distribution and of their origin in the past. These 

 have become classical and the basis of all subsequent speculation. The 

 " general indications," which, as Bentham pointed out, we owe to Hooker, 



