568 HISTORICAL GEOLOGY 



not the existing genus, at least a closely related one lived in the 

 Paleozoic. The impressions of the leaf, seeds, and male cones of Ju- 

 rassic trees are very similar to those of the trees now living (Fig. 528). 



Angiosperms. — The flowering plants are, at present, the common- 

 est of all plants, four sevenths of the existing species belonging to 

 this class. They have, however, a much shorter known history than 

 the conifers and various other groups, since no positive evidence 

 is at hand of their existence prior to the Lower Cretaceous. At the 

 beginning of this period, the horsetails, cycads, conifers, and ferns 

 were the common and conspicuous forms ; but, before its close, flower- 

 ing plants of both divisions (monocotyledons, represented to-day by 

 palms, lilies, and grasses, and dicotyledons, of which the elm, rose, 

 and clover are examples) had become prominent. The sassafras, fig, 

 willow, magnolia, tulip tree, laurel, and others have been recognized. 

 In the Upper Cretaceous the flowering plants became even more 

 conspicuous, and are represented, among many others, by palms, 

 beeches, birches, chestnuts, and poplars. 



The introduction of flowering plants was, perhaps, the most impor- 

 tant and far-reaching event in the whole history of vegetation, not 

 only because they almost immediately became dominant, but also 

 because of their influence upon the animal life of the succeeding peri- 

 ods. Hardly had flowers appeared, before a great horde of insects 

 which fed upon their honey or pollen seem to have sprung into exist- 

 ence. The nutritious grasses and the various nuts, seeds, and fruits 

 afforded a better food for non-carnivores than ever before in the his- 

 tory of the world. It was to be expected, therefore, that some new 

 type of animal life would be developed to take advantage of this 

 superior food supply. As we shall see in the discussion of the Ter- 

 tiary, the mammals, which kept a subordinate position throughout 

 the Mesozoic, rapidly took on bulk and variety and acquired posses- 

 sion of the earth as soon as they became adapted to this new food, 

 quickly supplanting the great reptiles of the Mesozoic. 



The flowering plants (angiosperms) had their origin, as far as is 

 known, on both sides of the North Atlantic during the Lower Cre- 

 taceous. Some of the earliest of these are somewhat generalized, 

 but do not give a positive clue to the group from which they were 

 descended. Just when and where they began we do not know, but 

 once started they spread rapidly and widely, and before the close of 

 the Lower Cretaceous had reached California, Alaska, Greenland, and 

 Bohemia, 



