CH. viii] DIFFERENTIATION 73 



mutation than this might have changed the whole of it to another 

 species. Then again, if a geological catastrophe come along, it 

 may easily destroy a whole species, or even genus, that has not 

 yet been able to spread far enough to get beyond its range. Unless 

 a fossil is found to cover such an area that it is unlikely that such 

 a fate may have overtaken its living representatives, it seems to 

 the writer in the highest degree unsafe to look upon it as an 

 ancestral form of existing species. It is more likely to be a lateral 

 mutation thrown off from the main line, and exterminated as a 

 genus by some happening. 



Lastly, there should be mentioned the all but complete absence 

 of transition stages in the fossils, a fact which violently disagrees 

 with the supposition that evolution was gradual and continuous. 



