

138 THE LIVING WORLD. 



general increase in complexity even in the case of 

 these types is indicated by two facts at least. We 

 see that in early periods it was the lower orders that 

 were most abundant and widely diversified, while 

 to-day it is the higher forms that are predominant. 

 For instance, among the Articulata it was the low 

 trilobites and cockroaches that existed in abundance 

 and diversified profusion in early times, while to-day 

 it is the higher orders that are the most abundant, 

 even though in the insects the lower orders are still 

 in existence. So in the case of the echinoderms, the 

 most abundant order of early ages was the Crinoidea 

 although the other classes of echinoderms were all in 

 existence. To-day though the crinoids are still in 

 existence, it is the higher orders that are the most 

 abundant. 



The second fact indicating the general advance is 

 the greater diversity of life to-day than in the earliest 

 times. Some groups have indeed passed into de- 

 cline or disappeared, but most of those that do exist 

 are to-day in greater profusion than in any of the 

 past ages. Taken as a whole, it is one of the most 

 evident teachings of the history of life that there 

 has been through all the ages a constant increase 

 in the profusion of living things, and a continually 

 growing diversity of form. Even though it is possi- 

 ible to say that many of the families are really no 

 more highly developed than were their represen- 

 tatives in the past, still the fact of the increase of 

 diversity of type is a plain indication of a general 

 advance. Thus the fact of the evident advance in the 

 structure of the representatives of some types, the 



