196 Prof. Qwen on the Class Mammalia. 
high grade of organization. Not any of the mammalian genera 
of the secondary periods occur in the tertiary ones. No genus 
such later and less typical Mammalia do more effective work by 
their more adaptively modified structures. e Ruminants, @. 9., 
tantly prolonged jaws could have been. 
Much additional and much truer ee, oh has, doubtless, been 
gained into the natural grouping of the 
the debris of a Broup, known at a subsequent period to be if 
er, 
respects the number and variety of the families, genera, 20 ee 
cies of such orders, because the paucity or multitude of instan no 
manifesting a given modification or grade of structure 1D © 
essential degree affects the value of such grade or modificatio™ 
2 
