86 UNDERHILL : 



geological domains of the marine invertebrates, fishes, reptiles 

 and mammals, brings to mind the question : What are the 

 forces working through these inconceivably long reaches of 

 time that build from insignificant beginning a faunal empire, 

 which in its turn is crowded out by the ascendanc}^ of another ? 

 Darwin undertook to solve the problem by showing that it 

 results from extermination according to a standard of fitness ; 

 nature's selection for survival of such individuals of a species 

 as possess variations better fitting them to a changing envi- 

 ronment. It is not for this article to comment upon the argu- 

 ments that have gathered around this thesis ; as to descent we 

 cannot in this day speak of it as a theor\^ ; descent is now 

 accepted by science as a fact — a law, and is no longer a 

 theory. But the doctrine of descent, or organic evolution, 

 was pre-Darwin ; Darwin brought to its support the results of 

 his prolonged study and observation, and, while recognizing 

 other influences, made clear the far-reaching force of natural 

 selection as a cause. How far this may be accepted in 

 explaining descent remains debatable, and, essentiall}^ the 

 deeper we go into the sciences relating to the subject the more 

 competent we become to weigh the evidence and reflect critic- 

 ally upon the arguments for or against Darwin's conclusions. 



In closing this brief survey of the rise of the mammals, 

 however, certain prominent influences for their success should 

 be mentioned. Though the members of the class first known 

 to appear were so insignificant in size and numbers as com- 

 pared with their huge saurian contemporaries, in two respects 

 they were superior ; in limbs and feet with a joint construc- 

 tion mechanically advanced, and in a superior brain. These 

 were the prime winning factors that preserved these little 

 creatures for their triumphant destiny, and which were further 

 and further elaborated upon through vast stretches of geolo- 

 gical time, until at the beginning of the Ceno/.oic era we 

 have, with the receding of the reptiles, the dawn of a new 

 life — the Age of Mammals. 



While there has been much branching and specialization 



