BOTANICAL SUPPOKT FOE DAEWIN 507 



means of distribution have overcome impediments and the 

 power to vary is shared equally by the different classes. 



A resume of the effects of physical conditions on plants 

 leads to discussion of the problems suggested by the traces of 

 world-wide migration of polar and cold temperate forms left 

 on the mountains, even in the tropics, and by the outlying 

 Oceanic islands ; present geological conditions are insufficient 

 to account for these. 



At the same time, the earliest known fossil plants are so 

 high in development already that subsequent evolution of 

 species cannot be said to support the doctrine of ' progressive 

 development ' — the doctrine, namely, that the course of 

 development is an advance from ' lower ' to ' higher.* 



Only be it said by way of caution [he characteristically 

 adds], we have no accurate idea of what systematic pro- 

 gression is in Botany, or the relation, progressive or retro- 

 gressive, between the simpler and more complex co-ordinates 

 in a group. 



From the sum of these theories, as arranged in accordance 

 with ascertained facts, he sets forth in § 35 his working ' assump- 

 tions ' of genealogical continuity since the earhest known 

 period ; the rise of differences through individual variation ; 

 their definition through the extinction of intermediates ; their 

 stability due to cross-fertilisation ; the temporary stability of 

 physical conditions, and the successful germination of those 

 seeds only which are adapted to these conditions. 



All these points are fundamentals in Darwin's theory. 

 That Botany, where no Lamarckian * effort ' could be predi- 

 cated, pronounced so plainly for the natural working of his 

 generahsations, was of the first importance. 



As to the choice between the opposed principles as working 

 hypotheses, neither can offer absolute certainty as to the origins 

 of things ; but while the one forbids the progress of enquiry, 

 the other opens the field to fruitful inference. 



As he puts it, in §§ 38-40, the arguments for the immutabihty 

 of species have neither gained nor lost by further investigation 

 and observation. The facts are unassailable that we have no 



