APPENDIX C 



EXTINCTION OF MAMMALS DURING THE GLACIAL 



PERIOD 



THE accompanying table shows the basis of Figure 22. The first 

 column shows the names of the geological periods from the 

 beginning of Tertiary times down to the present. The next three 

 show the length of the periods according to three diverse estimates. 

 Sollas' estimate represents the wholly inadmissible idea that the 

 various periods were of approximately equal duration. Matthews' 

 estimate is based on fossil evidence supplemented by the thickness 

 and character of the strata. Barrell has utilized these same data and 

 also certain newly discovered facts as to the rate at which uranium 

 and thorium lose their radioactivity and degenerate into a series of 

 other metals ending in lead. The views of all three authors, together 

 with references, are given in the paper by Barrell listed in Appendix 

 D. Barren's time-scale seems to the present writer the best yet avail- 

 able, but for our present purpose it makes no difference whether we 

 use the figures of Matthew or Barrell. 



In the columns headed "North American Genera" and "European 

 Genera," the first column in each case shows the total number of genera 

 of mammals listed by Osborn in his "Age of Mammals." The next 

 shows the number which became extinct, and the third gives the per- 

 centage of extinction. Then come three columns showing the average 

 percentage extinguished during one hundred thousand years according 

 to each time-scale. According to both Matthew and Barrell the rate 

 of extinction increased enormously during the Pleistocene or Glacial 

 Period. This is illustrated graphically in Figure 22, where the solid 

 line shows the rate of extinction according to Barrell, and the dotted 

 line according to Matthew. In interpreting Matthew's figures the 

 percentages at the right must be multipled by ten, while the names 



