7 z8 THE POPULAR SCIENCE MONTHLY. 



The tide of evolution rose ever higher and higher, without ebb, but it 

 nevertheless came in successive waves, each higher than the preceding, 

 and overborne by the succeeding. These successive cycles are the 

 dynasties or reigns of Agassiz, and ages of Dana ; the reign of mol- 

 lusks, the reign of fishes, of reptiles, of mammals, and finally of man. 

 During the early Palaeozoic times (Cambrian and Silurian) there were 

 no vertebrates.* But never in the history of the earth were mollusks 

 of greater size, number, and variety of form than then. They were 

 truly the rulers of these early seas. In the absence of competition of 

 still higher animals, they had things all their own way, and therefore 

 grew into a great monopoly of power. In the later Palaeozoic (Devo- 

 nian) fishes were introduced. They increased rapidly in size, number, 

 and variety ; and being of higher organization they quickly usurped 

 the empire of the seas, while the mollusca dwindled in size and impor- 

 tance, and sought safety in a less conspicuous position. In the Meso- 

 zoic times, reptiles, introduced a little earlier, f finding congenial condi- 

 tions and an unoccupied place above, rapidly increased in number, vari- 

 ety, and size, until sea and land seemed to have swarmed with them. 

 Never before or since have reptiles existed in such numbers, in such 

 variety of form, or assumed such huge proportions ; nor have they ever 

 since been so highly organized as then. They quickly became rulers 

 in every realm of Nature rulers of the sea, swimming reptiles ; rulers 

 of the land, walking reptiles ; and rulers of the air, flying reptiles. 

 In the unequal contest, fishes therefore sought safety in subordination. 

 Meanwhile mammals were introduced in the Mesozoic, but small in 

 size, low in type (marsupials), and by no means able to contest the em- 

 pire with the great reptiles. But in the Cenozoic (Tertiary) the condi- 

 tion apparently becoming favorable for their development, they rap- 

 idly increased in number, size, variety, and grade of organization, and 

 quickly overpowered the great reptiles, which almost immediately sank 

 into the subordinate position in which we now find them, and thus 

 found comparative safety. Finally, in the Quaternary, appeared man, 

 contending doubtfully for a while with the great mammals, but soon 

 (in Psychozoic) acquiring mastery through superior intelligence. The 

 huge and dangerous mammals were destroyed and are still being de- 

 stroyed ; the useful animals and plants were preserved and made sub- 

 servient to his wants ; and all things on the face of the earth are being 

 readjusted to the requirements of his rule. In all cases it will be ob- 

 served that the rulers were such because, by reason of strength, organi- 

 zation, and intelligence, they were fitted to rule. There is always 

 room at the top. To illustrate again by a growing tree : This succes- 

 sive culmination of higher and higher classes may be compared to 



* Fishes were first introduced in the later Silurian, but became dominant in the De- 

 vonian. 



f Amphibians were introduced in the Carboniferous, but true reptiles not until the 

 Permian. 



