116 
ORIGIN OF LIFE IN AMERICA 
and that for some reason or other they have subsequently 
been destroyed over wide areas, leaving at present only iso¬ 
lated colonies. In some instances this theory of the origin 
of discontinuous colonies of animals has been amply con¬ 
firmed by fossil evidence. Camels, for example, or we might 
say the family Camelidae to which they belong, are only 
represented in South America and Asia ; but since numerous 
fossil members of the family occur in North America, 
wo possess decisive evidence that long ago the range of 
the camel family was continuous. Discontinuity of range 
among mammals is always looked upon by zoologists as 
an unmistakable sign of antiquity. It is only when a 
similar range occurs among the more easily dispersed in¬ 
vertebrates and plants that- naturalists are in the habit 
of calling to aid exceptional forces of nature in explaining 
their origin. The undoubted facility with which human 
importations are scattered far and wide and become success¬ 
fully established in districts remote from their original 
home seems to encourage and invite speculations as to 
the origin of discontinuously distributed invertebrates of 
all kinds. Actual records of seeds, insects, snails, etc., sowed 
broadcast by accidental agencies far from their native 
land, appear to confirm the theories derived from successful 
human transplantations. So much are these in vogue that 
the ordinary and normal mode of dispersal is almost for¬ 
gotten. Winds no doubt exert an influence in driving species 
in the direction in which they blow. But many animals, 
beasts as well as beetles or butterflies, possess the faculty 
in a high degree of detecting the presence of their own kind 
by the sense of smell. Winds would, and do, as we know, 
convey scents from one animal to another, thus tending to 
bring the sexes together. This has been very clearly estab¬ 
lished by Mr. Webster.* The insect, being apprised by scent 
of the presence of its own kind, would endeavour to travel 
in the direction contrary to that of the prevailing wind in 
order to reach it. It may possibly be true that some insects 
are apt to spread in the direction of' the prevailing wind, as 
Mr. Webster asserts. But, judging from the few examples 
Webster, F. M., “Diffusion of Insects,” p. 797. 
