80 THE SEA AS A BARRIER CHAP. 
company on the highlands of the Cordillera in Peru and Keuador, 
but as we go farther south the latter are found on the plains of 
southern Argentina and Patagonia, as well as on the island of 
Tierra del Fuego at the sea level. Here then is a clear proof 
of the intimate connexion existing between temperature and 
station; the guanaco being an animal which can only live in 
cold or temperate climates, finds suitable conditions for its 
existence in tropical latitudes solely at a height of so many 
thousands of feet, although farther south it is able to thrive at 
the sea level.” This, however, cannot be pushed too far—the 
world cannot be mapped out into areas bounded by parallels of 
temperature as was once attempted—since there are plenty of 
cases like that of the Tiger, which is as much at home in a 
tropical jungle as on the icy plains of Northern Asia. 
Seeing that there are in many cases no climatic barriers to 
the spreading of a given race of animals over a larger area of 
distribution than it actually occupies, it becomes important to 
inquire why there are so many cases of restriction in range. 
It is possible to see, at any rate, three causes which are 
responsible for a large number of such cases. In the first place, 
a given species of animal must have originated at a certain spot ; 
its multiplication in individuals must always be a slow matter, 
since enemies, and untoward events generally, would conspire 
to check the natural multiplication by geometrical progression. 
A long time might therefore elapse before the species greatly 
extended its range. <A restricted distribution may therefore, 
Im some cases, mean a modern race. In the second place, 
there are definite physical barriers which check the migra- 
tion of species. The terrestrial Mammalia cannot cross wide 
arms of the sea; that they can and do swim for considerable 
distances has been proved in several instances; but, as has 
been pointed out, it is unlikely that a purely terrestrial mam- 
mal would voluntarily swim out into an unknown sea. And 
then if it did, and successfully reached the opposite side, 
nothing would happen unless it were a pregnant female; or, if 
not pregnant, till a male swam very soon afterwards in exactly the 
same direction. Many travellers have told of floating islands, 
formed of torn-up trees and brushwood, which have been seen 
at the mouths of large rivers, with animal passengers upon them. 
These are, however, so much at the mercy of currents and storms, 
