10 HABIT AND INTELLIGENCE. 



Contrasted species by a process of gradual change. lu geological 

 lo^t^icaT"' history, on the contrary, there is nothing in the slightest 

 histoiy. degree analogous to these lixed laws of development. 

 There is no genetic process. Each event of the history is 

 determined, not from within, as it is in vital development, 

 but from without ; or, in other words, each event is deter- 

 mined, not by the event which has gone immediately 

 before it, but by events the cause of which has no con- 

 nexion with the series under investigation. Thus, for in- 

 stance, let us suppose a stratum of limestone to be depo- 

 sited in a shallow sea. What formation is to come next ? 

 This is not in any way determined by the nature of the 

 limestone ; it is determined by causes which act from 

 without, and are in no way connected with the character 

 of the strata already deposited. If the bed of the sea 

 continues to subside about as fast as the limestone is de- 

 posited, strata of limestone may continue to succeed each 

 other for an indefinite time. If, on the contrary, it is raised 

 so as to convert the limestone stratum into dry land, an 

 indefinite time may pass without any deposit whatever 

 being formed over it. If the limestone continues to be 

 covered by a shallow sea, while a change of the levels in a 

 neighbouring continent causes a great and muddy river to 

 flow into that sea, the mud will form a stratum lying over 

 the limestone, and of a mineral character totally diffe- 

 rent from it : and if, what is a very probable occurrence, 

 great spaces of very shallow water are kept free from 

 mud by means of dense borders of aquatic vegetation, 

 then, if tlie conditions of climate are favourable, these 

 spaces will themselves be overgrown with vegetation, 

 which, dying, will produce beds of a peaty substance : 

 and subsequent geological changes may transform this 

 into coal. And all these strata may be mechanically 

 broken up and contorted by volcanic action, while at the 

 same time they are chemically metamorphosed by the 

 volcanic heat. 



These examples will, I think, make my meaning clear 

 when I speak of the difference between a genetic process 

 and a simply historical one. A genetic process may be 



