APPENDIX C 269 



reckoned as extinguished during that period. Since practically all 

 living forms are known, however, there is almost no chance that any 

 of the Pleistocene forms now reckoned as extinct will be transferred 

 to the opposite category. Hence while the percentage of apparently 

 extinct forms will steadily decrease in earlier periods, it will change 

 but slightly in the Pleistocene. Accordingly, as knowledge increases, 

 the earlier, or left-hand portions of Figure 22 will fall steadily lower, 

 the greatest fall being in the earlier periods. Thus it seems safe to 

 say that the rate of extinction of mammals during the last Glacial 

 Period was fully four times as rapid as the average during the pre- 

 ceding Tertiary Era. 



