116 OKIGIN 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, 

 we 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 dis continuously 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. 



