16 Zoology of Colorado 



At the end of the Mesozoic the dinosaurs disappeared, and 

 the true Age of Mammals began. But we do not find in the 

 western rocks a record of continuous progress and development. 

 Instead, we have evidence of successive waves of invasion, new 

 types which had evolved elsewhere coming in to supplant the old, 

 much as the white man has supplanted native tribes in various 

 regions, or as modern man (Homo sapiens) supplanted the Nean- 

 derthal man ages ago in Europe. Owing to these invasions, and 

 the comparatively rapid evolution of the mammalia, the bones of 

 these animals afford us more accurate means of discriminating 

 between successive strata than any other fossils. It is thus of 

 great interest to collect all the materials available, both to demon- 

 strate evolution and reach a better understanding of geology. The 

 mounted specimens in the Colorado Museum in City Park, Denver, 

 give us a lively realization of the former mammalian life of Colo- 

 rado, and appeal to the imagination of all intelligent persons. 

 Looking at them, we readily understand that in addition to the 

 invasions and evolution already mentioned, there is another matter 

 to consider, namely extinction. Genera and families, even orders, 

 have gone out as well as come in. Thus we have additional 

 date lines for the identification of strata. This coming in and going 

 out may have often taken quite short periods, as we know to have 

 been the case within historic times. Thompson Seton has given 

 us a vivid account of the destruction by disease of the rabbits in 

 the Mackenzie region of British America. Where millions existed 

 recently, he could not find a single individual. Consequently the 

 Canada lynxes, which had multiplied on a rabbit diet, were actually 

 reduced to starvation. In such cases it is generally found that a 

 few animals survive, and eventually the species which reached so 

 low an ebb is restored to at least temporary prosperity. But as 

 we know well from the records of the past, this does not always 

 happen, and many an interesting animal has disappeared, never to 

 return. Can we be absolutely sure that man himself will not 

 suffer such a fate? The world-wide influenza epidemic, which 

 killed more than the great war, may be taken as a warning. Among 

 the mammals of which we have a fairly complete evolutionary 

 history, the horse group stands pre-eminent. This is particularly 

 satisfactory because the horse is really a very peculiar animal, 

 the origin of which, in the absence of direct information, would 



