30 LECTURES 



which takes place every year, and that for the entire 

 width and breadth of the continent. 



As another means of the dispersal of animals we have 

 those various movements which take place in the atmos- 

 phere itself, and it has been noted that the trade winds, 

 hurricanes and cyclones, or even the minor whirlwinds 

 do much sometimes toward scattering the lighter forms 

 of animal life as well as the seeds of many plants, and by 

 such means new organisms are often transported to 

 distant faunal areas to which they were formerly un- 

 known. In any special case, should the environment 

 prove compatible to a species thus violently introduced, 

 that species may become established and thrive in the 

 new region, or it may, the reverse holding true, become 

 gradually extinct, or even may perish at once. 



Q.uite a number of artificial means, introduced by man, 

 himself, have from time to time acted as factors in either 

 favoring the dispersal of certain animals or tending to 

 restrict them to certain localities. Bridging large 

 streams, the transportation of many of the smaller forms 

 in steamers, grain cars, trains, and other modes of con- 

 veyance; the joining of large bodies of water by means of 

 the opening of artificial canals; and finally, even the con- 

 struction of country roads have all had their influence 

 along the lines indicated. 



Anticipating for the moment the discussion of the main 

 subject of our lecture, we mav say here that the dispersal 

 and rearrangement of faunae and floras in past ages of the 

 world, have been powerfully affected by geologic changes; 

 as, for example, the effects produced by the glacial 

 period as a whole, as well as those profounder changes 

 the upheavals, depressions and submergences of conti- 

 nental margins and areas. It must be borne well in mind 

 that at the present time the faunae and florae of the various 

 zoogeographic legions are continuously but gradually 

 undergoing many changes, brought about by the ex- 

 termination of some forms, by the slow development of 

 new species, and by the gradual physical changes of the 

 earth characteristic of the present epoch. 



To sum them up, we may say, in tne words of. Huxley: 

 "It has been discovered by careful comparison of local 

 faunae and florae that certain areas of the earth's surface 

 are inhabited by groups of animals and plants which are 

 not found elsewhere, and which thus characterize each 

 of these areas. 1 ' 



We are now in position to return to the naming of those 

 areas, and as we have already said, the views of Mr. Wai- 



