XI.] SPECIFIC GENESIS. 251 



ually accumulntiiig or diversely combining actions of all 

 these on liiglily-sensilive structures, Avliicli arc themselves 

 possessed of internal resj)onsive powers and tendencies, 

 may well result in occasional repeated productions of forms 

 harmonious and vigorous, and differing from the parental 

 forms in proportion to the result of the combining or con- 

 flictinn: action of all external and internal influences. 



If, in the past history of this })lanet, more causes ever 

 intervened, or intervened more energetically tlian at pres- 

 ent, we miglit a priori expect a richer and more various 

 evolution of forms more radically differing than any which 

 could be produced under conditions of more perfect equi- 

 librium. At the same time, if it be true that the last few 

 thousand years have been a ])eriod of remarkable and 

 exceptional uniformity as regards this planet's astronomical 

 relations, there are then some grounds for thinking that 

 organic evolution may have been exceptionally depressed 

 during the same epoch. 



Now, as to the fact that sudden changes and sudden 

 developments have occurred, and as to the probability that 

 such changes are likely to occur, evidence was given in 

 Chapter IV. 



In Chapter V. we also saw that minerals become modi- 

 fied suddenly and considerably by the action of incident 

 forces — as, e. g., the production of hexagonal tabular crys- 

 tals of carbonate of copper by sulphuric acid, and of long 

 rectangular i)risms by ammonia, etc. 



We have thus a certain antec(Hlent j)robability that if 

 changes are produced in specific manifestation through inci- 

 dent forces, these changes will be sensible and considerable, 

 not minute and infinitesimal. 



Consequently, it is probable that new species have 

 appeared from time to time with comparative suddenness, 

 and that they still continue so to arise if all the conditions 

 necessary for specific evolution now obtain. 



