PLAN AND PURPOSE IN NATURE. 205 



Cretaceous period. Again in the opening eras of the 

 Eocene and in the Miocene warmth of ch'mate and much 

 moisture prevailed. Higher hfe of animal and plant flour- 

 ished abundantly on the continents now largely increased 

 in area by elevation since the later Mesozoic times. The 

 excessive moisture of the latter diminished, specialization 

 of climate increased, lower temperatures gradually prevailed. 

 A force, which is never a convenient one for strict uniformi- 

 tarians either in geology or palaeontology to entertain, 

 became remarkably prominent in the later Miocene times. 

 In quiet Mesozoic ages volcanic action, though existing 

 in all ages, was little pronounced. But in Cainozoic times 

 it became of immense importance in the making of mountain 

 ranges and valleys. It is difficult to say whether the results 

 of its working upon the face of the earth, or the restraint of 

 its power, is the more remarkable. The effects at least were 

 of supreme importance to more specialized groups of inhabi- 

 tants, and in due time the geography of the earth and sea 

 came slowly to its present limits, and in this result volcanic 

 action found its beneficent purpose. The important remark 

 of Sir William Dawson must here be borne in mind in the 

 study of environments adapted to coming requiremeuts :* 

 " We also see that, not the adverse conditions of straggle 

 for existence, but the favouring conditions of scope for 

 expansion, were, as might rationally be expected, the accom- 

 paniments and secondary causes of new inbursts of life." 

 This principle is seen carried out in the equable and com- 

 paratively uniform character of the home into which the 

 Cambrian and Silurian invertebrates were introduced; in 

 the warm marine environment with teeming supplies of food 

 which greeted the Devonian fishes, the moist marshy and 

 mild terrestrial climate for the flora of those days, the 

 restrained action of volcanoes and increasing emergence 

 of land. How suited were such free and luxuriant con- 

 ditioDs of life to the marvellous fauna and flora peculiar 

 to the Mesozoic period ! The value of the Sub-Carboniferous 

 period of that era, with its long submergence of the land in 

 shallow water, paving the way for the luxuriant land vege- 

 tation of the Carboniferous period, and the significant 

 introduction of vast quantities of insects pregnant of 

 benevolence for future plant life, withal unconscious of 

 their honoured position, may be borne in mind in illus- 



* Modern Ideas of Evolution, p. 118. 



