FROM THE STANDPOINT OF AN IDEALIST. 445 
Yet it may be adaptive in another sense—namely, in its response to the 
remnant of the conditions of an age long passed away. One might regard 
it as the last kick of the organism in response to what remains of the 
primeval uniformity of conditions, its last effort to break through the ever- 
contracting ring of the differentiating agencies. The farther we go back 
the greater is the capacity for mutations and the greater will be the 
mutations ; and it is argued that they ought to be more frequent and more 
extensive in plant-groups of large than of small range. A large mutation 
would usually be impossible nowadays except under conditions approaching 
those of the early ages in their uniformity. One would look for some 
approach to those times in the dense forests of tropical lowlands and in the 
forests of the cloud-belt or rain-zone on tropical mountains. 
The ascription of periodicity to mutability by De Vries is well known, 
and appeals to this principle in elucidation of the rapid rise of the Angio- 
sperms in the Cretaceous period have been made ; but the question, why 
basic characters so mutable then are stable now, always remains. The 
position is well put by Harshberger in his great work, *The Phytogeo- 
graphic Survey of North America, p. 173, 1911. “If this periodicity of 
mutation (so he writes) is recognized as an evolutionary principle, we have 
a reasonable explanation for the sudden appearance of so many new forms 
during the Cretaceous period, for during this stage of the development of 
the vegetable kingdom, through causes yet unknown, the progenitors of the 
existing phanerogams were in a high state of mutability." 
A few remarks may here be made on the relation of the views of distri- 
bution here advocated to the Age and Area theory of Dr. Willis. If one 
for the moment ignores his adoption of the prevailing practice of building 
up a family from the species, there is but little that is essentially inconsistent 
with the differentiation hypothesis. Had he formally associated with his 
Age and Area principle the twin principle of Rank and Range, and. all that 
it implies, he could not have avoided coming into line in this matter. 
Since he extends his views to the larger groups, his Age and Area theory 
is of. general application, and his conception of the distribution of families, 
apart from his standpoint of their genesis, might very well have been 
acquired in a line of argument favouring the views supported in this paper. 
His discussions of the Dilleniacez, Menispermaceze, and Podostemaces are 
cases in point. (See Ann. Roy. Bot. Gard. Peradeniya, 1902, 1907; 
Phil. Trans. Roy. Soc. Lond. 1915 ; Proc. Roy. Soc. 1914; Ann. Bot. 1915, 
ete.) Thus he connects the origin of these three pantropical families with 
the most primitive and most widely distributed genera— genera that almost 
possess the range of the families. This is differentiation pure and simple. 
Then again, though he departs from the principle of differentiation when 
he endeavours to find the original centre of dispersion or home of the 
Dilleniacexe, he comes very near it in the case of the Menispermacez in his 
