CONGRUITY OF THE EVIDENCE 307 



lapse of ever-fruitful ages ; though the descendants or ultimate 

 ramifications of other of these trees— dating back to quite different 

 and perhaps far-distant epochs— may still survive upon the earth's 

 surface. How far, hovi^ever, those roots from which the existing 

 higher forms of life are derived, may have extended back into the 

 depths of geologic time, we are utterly unable to estimate.' 



It is only natural to expect in accordance with these views that, 

 while more or less similarity would be likely to exist between the 

 lower forms of life which have appeared at different periods of the 

 earth's history, more and more divergence might be encountered 

 amongst such higher aquatic or aerial types whose ancestors may 

 have lived through long geologic ages. Among such forms, con- 

 siderable diversity may be induced by the operation of the ordinary 

 factors of evolution. So that if the descendants of similar organisms 

 (derived perhaps from totally independent stocks) have been 

 exposed to notably different external conditions in different ages ; 

 or if in any of them modifications have otherwise arisen, the forms 

 ultimately produced along such lines of development may be widely 

 different from one another, although belonging to similar types. 



And at different periods in the earth's history, specialisations, now 

 of one type and now of another, have been more and more 

 manifest or dominant. In the Silurian epoch, strange crustacean 

 Trilobites abounded in all the seas. In the Devonian epoch fishes 

 of a remarkable structure were most plentifully represented ; while 

 in the earlier Carboniferous period the cup-like Encrinites existed 

 in abundance, though in the later portions of this epoch they 

 were altogether thrown into the shade by that vast tropical 

 vegetation from which we now derive our supplies of coal. In 

 the Oolitic period, or so-called ' age of reptiles/ we have a most 

 remarkable abundance of Saurian forms, and the Amphibian type 

 reached its highest development. Huge Ichthyosauri and Plesio- 

 sauri swam in the lakes and rivers, whilst strange and gigantic 

 winged Lizards mounted into the air. In the later Tertiary period 



' Cope in his work, "The Primary Factors of Organic Evolution" (1896), 

 dwells upon the fact " that the phylogenetic lines have not been continuous " ; he 

 says, " the point of departure of the progressive lines of one period of time has 

 not been from the terminal types of the lines of preceding ages, but from points 

 further back in the series." And again he says, " Many lines of variation have 

 been at one geologic period and another discontinued. It is also true that certain 

 divergencies from the main lines have appeared. . . . Such variations do not 

 seem to have had any material effect on the general Course of Evolution " 

 (pp. 172-222). 



