622 THE BEGINNINGS OF LIFE. 



winged Lizards mounted into the air. Whilst in 

 the later Tertiary period we find the Mammalian 

 type exhibiting remarkable divergences from previously- 

 existing forms; first by the appearance of innumerable, 

 huge Mastodons, Megatheriums, and other unwieldy 

 denizens of the ancient forests and plains ; and subse- 

 quently by the gradual modification of one of the rami- 

 fications of the Quadrumanous order, into those beings 

 from whom primeval Man himself may claim to have 

 been evolved. 



These several types of life, however, which have 

 from time to time become more and more specialized, 

 do not in any sense represent the members of one 

 progressive series. They are rather the products of 

 different evolutional divergences, taking place now in 

 one direction and now in another. 



Our knowledge of the various living forms which have 

 existed in past ages is, indeed, of the most fragmentary 

 character : first, on account of the unequal or imperfect 

 manner in which the several forms may be represented 

 in the strata pertaining to the period j secondly, on ac- 

 count of the extremely limited nature of the explorations 

 which have been made in these imperfectly representative 

 strata ; and, thirdly, because so many parts of the record 

 are absolutely inaccessible to us nearly all beneath the 

 Silurian system 'having been blotted out by time, whilst 

 those two-thirds of the earth's surface in which the 

 remaining strata are to be found are now covered over 



