L SELECTION.‏ .تہ 
IN his introduction, Mr. Darwin tells us that “when on 
board H. M. S. Beagle as naturalist, I was much struck 
with certain facts in the distribution of the organic beings 
inhabiting South America, and in the geological relations 
of the present to the past inhabitants of that continent. 
These facts seemed to throw some light on the origin of 
species.” In the chapter on Geographical Distribution, he 
says that "neither the similarity nor the dissimilarity of 
the inhabitants of various regions can be accounted for 
by their climatal and other physical conditions." Thus,the 
plants and animals of South America, between latitudes 
25? and 55?, are very different from those of Australia and 
South Africa; and yet the physical conditions of these three 
countries, within these limits, are very similar, while, not- 
withstanding the great differences of the physical conditions 
north of 25? and south of 35?, the plants and animals of 
these parts of South America are very similar. The 
existence of lofty mountain-chains, great deserts, etc. acts 
as a barrier to the free dispersion of plants and animals, and, 
therefore, is of great importance in reference to Geograph- 
ical Distribution; thus, the life of opposite mountain-chains 
is often quite different. This is equally true of the ocean 
life on opposite sides of a continent: thus, the marine 
animals of the north side of the Isthmus of Panama are 
very different from those of the south side, whereas similar 
fishes are found in as remote waters as the Pacific and Indian 
(141) 
