630 HISTORICAL GEOLOGY 



tinguishable from species inhabiting Europe to-day. That some of 

 them were herders of plant lice is proved by blocks of amber con- 

 taining masses of ants mingled with the plant lice which they were 

 attending when the liquid resin of the Oligocene pines flowed over 

 and embedded them. Possibly the soldier cast is a recent innovation, 

 but the differentiation of the males, queens, and workers was as 

 extreme, and precisely of the same character as now." (W. M. 

 Wheeler.) 



The insects of these Colorado Miocene deposits indicate that the 

 locality was doubtless an upland or mountain with a warm, moist, 

 but not tropical climate. The presence of such a climate in the 

 higher lands of this epoch does not preclude the possibility of arid 

 conditions on the Great Plains and in Texas (p. 635). 



There appears to have been but little important change in the 

 insect world since the middle of the Eocene or earlier, almost no new 

 orders or even families having appeared, although the genera and 

 species have changed. This is perhaps not surprising, since insects 

 of the modern type made their appearance soon after flowering plants 

 became widespread, and had consequently perfected their structure 

 and organs previous to the Tertiary, during the long Cretaceous 

 Period. 



REFERENCES FOR INSECTS 



Osborn, H. F., — Age of Mammals, pp. 263, 450. 

 Zittel-Eastman, — Textbook of Paleontology, Vol. 1, pp. 794-821, 



Vegetation 



The vegetable kingdom reached its culmination before the animal 

 kingdom ; and as far as plant evolution is concerned, it is almost 

 arbitrary to separate the Tertiary from the Mesozoic or from the 

 Pleistocene. Even at the beginning of the Tertiary, the general 

 aspect of the forests was not very different from that of to-day, as 

 the presence of maples, poplars, sycamores, walnuts, hazelnuts, 

 elms, yews, cedars, and sequoias (redwoods) shows. The associa- 

 tion, however, is rather remarkable since with the above are found 

 figs and palms. The presence in Greenland, Iceland, and Spitz- 

 bergen of trees such as now grow in the United States indicates a 

 warmer climate in the polar regions. 



Grasses. — There was, however, one very important element of 

 the vegetation — the grasses — which at the beginning of the period 



