FOSSILS— THEIR NATURE AND INTERPRETATION. 89 



system ; ^ of the genera have a life-period of one or two sys- 

 tems length ; ^'y of them lived only through two periods and 

 into a third ; and only 9, or -^-^, continued existence for more 

 than the length of three geological systems. 



Organisms express Evolution in their Geological History ; a Fun- 

 damental Law. — These statistics are chosen only as a conven- 

 ient illustration of a general law, which might be illustrated 

 by any other group of which we have the facts. Without 

 stopping to ascertain what the particular nature of the forms 

 is, it is evident that divergence of organic form is intimately 

 associated zuith lapse of time. We do not require to see every 

 form that has lived on the earth to distinguish the working of 

 this law ; but the few imperfect evidences, as well as the fuller 

 particulars we know respecting some of the better preserved 

 organisms, emphasize the presence of the law whenever we 

 examine the facts. Thus we are led to conclude that mor- 

 phological differentiation [evolution^ is as characteristic of the 

 history of organisms in geological time as organic grozvth {de- 

 velopment) is characteristic of the history of the individual 

 organism in its lifetime. 



The Meaning of Genus and Species. — We have been speaking 

 of combinations of form which are defined as classes, orders. 



Fig. 6.^Favosiies niagarensis. Hall. Original figures of the fossil coral from the limestone on 

 Goat Island, in Niagara River : a. fragment of the coral showing the ends of the corallites ; b. 

 a magnified view of two corallites, showing the dissepiments and the perforations of the walls ; 

 c, end view of the corallites, showing the walls and perforations. (After Hall.) 



genera, or species, and of genera as living at a particular time, 

 and having a particular range, and differing one from another. 

 In the study of fossils w^e do not actually see species and 



