290 THE SURVIVAL OF THE UNLIKE. [XVII. 



in the ages to come, the early flowers of to-day will 

 disappear, to be replaced by what are now onr later 

 flowers, whose place, in turn, will be filled by forms 

 that are yet to be," All this is offered as a mere 

 suggestion, but there is no hint given us as to the 

 reasons for an assumption that the floral epochs con- 

 stantly tend to become earlier. I assume that Mr. 

 Clarke means to say that the climatal seasons are 

 becoming shorter, and are thereby crowding back the 

 older forms, but the idea is not expressed. May not 

 the phenomena be better explained upon Darwin's 

 hypothesis of the "divergence of character," by virtue 

 of which a new form occupies places unoccupied by 

 existing forms ? We may assume that the primitive 

 plants found the field clear, so to speak, and developed 

 themselves at once upon the arrival of congenial con- 

 ditions ; but later forms, finding the earth occupied 

 early in the season, were obliged to push on and make 

 their greatest development of vegetation later in the 

 season. And specialized types are those which have 

 boldly reached out and have appropriated places of 

 least resistance. 



But whatever evolutional explanation may be given 

 of the origin of the periodical epochs of plants, it 

 nevertheless remains that these epochs are evidentl}' 

 adaptations to environments, of which climate is always 

 the chief factor. We may still seek, therefore, for the 

 physiological constant. It is generally conceded that 

 temperature, or relative heat, is more intimately asso- 

 ciated with the periods of plants than any other factor 

 of climate, and the physiological constant has been 

 sought in this direction. One of the earliest American 

 expressions upon this point was made in 1859 by 



