BOTANICAL SUPPORT 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 earhest 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 stabihty 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 immutability 
of species have neither gained nor lost by further investigation 
and observation. The facts are unassailable that we have no 
