51 R. C. REID ON THE NATURAL HISTORY OF ISOLATED PONDS. 273 
still living in Britain, but a large proportion of the mammals are 
now extinct. Next came a cold period, when the inhabitants of 
the Arctic regions moved southward, and took possession of a 
country no longer lit for forest-trees or southern animals. Finally, 
the cold became less, the Arctic species retreated northward, and 
were replaced by an assemblage similar in many respects to that 
5vhich inhabited the country previous to the Glacial epoch. 
Continual change is traceable in the organic world throughout 
all geological periods, and the present forms no exception to 
the rule. 
When we realise the ceaseless migration which is always going 
on in the organic world, the enquiry is forced upon us, How did 
tho existing fauna and flora enter this country, and what facilities 
has each of the species for dispersal into new regions 1 It is no 
longer sufficient to answer, as in the old days, that this species is 
“ native,” and that other is “ introduced,” for we soon learn that in 
those regions which were affected by the great waves of cold and 
heat there is no such thing as an aboriginal species. All our 
animals and plants are hero, not by reason of this country being 
their original home, but because they are species that were able to 
seek new homes when the climate changed, or are species that 
could spread rapidly into new districts. 
Tho British fauna and flora is not insular. It is made up of 
winners in the race, and survivors in the struggle, for an un- 
occupied country ; since its introduction too short a time has 
elapsed for many insular modifications to appear. Our flora, being 
made up of recently successful competitors in such a race, tends to 
spread far and wide. The plants of true oceanic islands seldom 
establish themselves in new regions, for a truly insular assemblage 
consists largely of locally-modified forms which are now little 
adapted for dispersal, for dispersal with them meant only waste 
and destruction. 
So often have the fauna and flora of this country changed, that 
wo are led to wonder whether geological time can be sufficiently 
long for the process ; or, on the other hand, whether most species 
may not possess greater facilities for dispersal than is usually 
imagined. The constant recurrence of such questions, and the 
conviction, after close study of Pliocene and Pleistocene natural 
history, that changes in the organic world are often, in all probability, 
