ON BIOLOGY. 51 



rivers was greatly increased, and thus were formed those 

 deep canons in the regions (New Mexico, Colorado and 

 Arizona), where the elevation was greatest. Thus the 

 down sinking of the Mid-Pacific bottom, the bodily up- 

 heaval of the Pacific side of the continent, and the down 

 cutting of the river channels into those wonderful canons 

 are closely connected with each other." 



It must not be understood that these vast depressions 

 and upheavals of great areas of territory were performed 

 in a rapid manner, for they required ages for their com- 

 pletion; indeed, In several parts of the earth very much 

 the same thing is going on under the very eye of science. 

 The coast of Norway, for example, is now rising at an 

 average rate of two and a half feet per century. Let 

 that continue for 1,000 centuries, and you may 

 easily understand what I mean by a geologic upheaval 

 of a land area. Still those great revolutions, notwith- 

 standing the gradualn^ss of their performance, did pow- 

 erfully affect in time the various faunae of the country, 

 and greatly influenced mammalian migration. In numer- 

 ous instances those migrations have been carefully 

 worked out and traced, traced as the several mammalian 

 faunae influenced by one cause or another passed from one 

 part of a continent to adjacent land areas. Those changes 

 in the habitats of entire groups of mammals went on in 

 obedience to certain ever-acting natural laws, for thou- 

 sands upon thousands of years, down to the present day. 

 Many types, as we have seen, became utterly extinct; 

 others by a slow evolution have left descendants in no 

 way resembling their original ancestors, while as a whole 

 the entire host of the world's mammalian faunae has led 

 up through those untold ages to quite perfectly explain 

 the laws of animal distribution as I have sketched them 

 for you to-day, and to thoroughly account for the pres- 

 ence in various parts of the earth of the animals as tney 

 now exist there. And, one other very important thing 

 must be noted, and it is that mammals, as has been the 

 case with most all other organized forms, since their first 

 appearance on the earth have passed from types of a 

 highly generalized structure, by evolution, to their des- 

 cendants of modern times wherein the structure is seen to 

 be far more highly specialized. This passage from the 

 simpler anatomical forms of mammals, all of which are 

 now extinct, to the more highly specialized types of the 

 existing faunas, has taken almost inconceivable ages of 

 time. 



What has just been said about the evolution of the 



