126 FACTS AND FANCIES 



in the arctic regions — periods when plants 

 such as now live in temperate regions could ^ 



enjoy six months of continuous sunshine — ' I 



were eminently favorable to the development 

 of such plants, and were utilized for the intro- 

 duction of new floras, which subsequently 

 spread to the southward. Thus we see phys- 

 ical changes occurring in an orderly succes- 

 sion and made subservient to the progress of 

 life. 



6. There is no direct evidence that in the 

 course of geological time one -species has been 

 gradually or suddenly changed into another. 

 Of the latter we could scarcely expect to find 

 any evidence in fossils ; but of the former, if it 

 had occurred, we might expect to find indica- 

 tions in the history of some of the numerous 

 species which have been traced through succes- 

 sive geological formations. Species which thus 

 continue for a great length of time usually pre- 

 sent numerous varietal forms which have some- 

 times been described as new species ; but when 

 carefully scrutinized they are found to be mere- 

 ly local and temporary, and to pass into each 

 other. On the other hand, we constantly find 

 species replaced by others entirely new, and 

 this without any transition. The two classes 



