APPENDIX C 257 



of the periods as given at the bottom must be crowded into the small 

 horizontal distance occupied by the dotted curve. Incidentally the 

 effect of a knowledge of radioactivity upon our conceptions of geologi- 

 cal time is evident from the fact that according to Sollas and most of 

 the earlier geologists our entire diagram would have to be crowded 

 into the space occupied by the Upper Pliocene (U) and the Pleisto- 

 cene (PI) according to Barrell. 



In studying Figure 7 the thoughtful reader may at once inquire 

 whether the sudden apparent increase in the rate of extinction during 

 the Glacial Period may not be due merely to the scantiness of our 

 knowledge of earlier times. The reverse is actually the case. The 

 less we know of the fauna of an early period the more probably its 

 component animals will appear to have become extinct. In general 

 the older faunas are less fully known than the later ones. This is 

 evident from the following table which shows the approximate number 

 of mammalian genera known in all parts of the world during each 

 period since the beginning of the Tertiary: 



At the beginning of the Eocene the number of genera may not have 

 been as great as in the Pleistocene, but surely the difference was 

 nothing like so great as is indicated by the figures 47 and 181 in Table 

 II. Hence the chances of finding new forms from the basal Eocene, 

 for example, are much greater than from the Pleistocene. The chances 

 are, however, that among these new forms the percentage that are 

 not found in the succeeding period and hence are considered to have 

 been extinguished in the earlier period will be the same as among the 

 forms already known. On the other hand, there is a very large chance 

 that among the many unknown forms of the Lower Eocene, for 

 example, there will some day come to light a considerable number that 

 have as yet been found only in the basal Eocene and are now wrongly 



