444 
DARWINISM 
CHAP. XIV 
The same remark applies to the views of Professor Geddes on 
the laws of growth which have determined certain essential 
features in the morphology of plants and animals. The 
attempt to substitute these laws for those of variation and 
natural selection has failed in cases where we can apply a 
definite test, as in that of the origin of spines on trees and 
shrubs ; while the extreme diversity of vegetable structure 
and form among the plants of the same country and of the 
same natural order, of itself affords a proof of the preponder¬ 
ating influence of variation and natural selection in keeping 
the many diverse forms in harmony with the highly complex 
and ever-changing environment. 
Lastly, we have seen that Professor Weismann’s theory of 
the continuity of the germ-plasm and the consecpient non¬ 
heredity of acquired characters, while in perfect harmony 
with all the well-ascertained facts of heredity and development, 
adds greatly to the importance of natural selection as the one 
invariable and ever-present factor in all organic change, and 
that which can alone have produced the temporary fixity 
combined with the secular modification of species. While 
admitting, as Darwin always admitted, the co-operation of the 
fundamental laws of growth and variation, of correlation and 
heredity, in determining the direction of lines of variation 
or in the initiation of peculiar organs, we find that variation 
and natural selection are ever-present agencies, which take 
possession, as it were, of every minute change originated 
by these fundamental causes, check or favour their further 
development, or modify them in countless varied ways 
according to the varying needs of the organism. Whatever 
other causes have been at work, Natural Selection is supreme, 
to an extent which even Darwin himself hesitated to claim 
for it. The more we study it the more we are convinced of 
its overpowering importance, and the more confidently we 
claim, in Darwin’s own words, that it “ has been the most 
important, but not the exclusive, means of modification.” 
