HOW SPECIES PERISH. 223 



posing. The further we go back into these by -gone ages 

 the greater is the difference between most of the species 

 living in them and those existing now. Not only species 

 appear and become extinct again, but vast numbers of 

 organic types have been evolved and then been exter- 

 minated. So gradually have these mighty changes 

 taken place, so insensibly have these various forms been 

 evolved, that in many cases their pedigree may be traced 

 with startling distinctness back into the distant past 

 back from representative types living on the earth to- 

 day to their early and vastly different ancestors. 



So far as we can judge, these stupendous changes in 

 organic life have been accomplished by a wonderfully 

 slow and minutely gradual process. Evolution is a work 

 of peace as well as an attendant of cataclysm ; its great 

 and wonderful mission is constantly in progress, but its 

 grandest results can only be discerned through the vista 

 of uncounted ages. The work of segregation progresses 

 so slowly and insensibly that but little of the process is 

 visible to living man ; nevertheless, the careful student 

 of nature is able to collect abundant evidence to show 

 that even at the present time all species are by no means 

 stationary ; on the contrary, some are being differentiated 

 from existing forms, others have only quite recently 

 become extinct, whilst some are slowly passing away. 

 So delicately adjusted are all species to their environ- 

 ment that the least disturbing element is apt to affect 



