290 PHYSIOLOGICAL BOTANY. PART II. 



of our globe, though incalculably earlier than any date 

 to which we can refer by authentic records. 



(297.) Oriyiit of I'dricdex. The origin of varieties 

 is a phenomenon in some respects analogous to the 

 creation of hybrids ; and it has been even supposed 

 that all races, or such varieties as are capable of main- 

 taining their peculiarities by seed, must have originated in 

 hybridity between two species. If such hybrids have 

 been fertilised by the parent species, and new hybrids of 

 the second and third degree been produced, these will so 

 closely resemble the parent plants that they will appear 

 to be mere varieties of it. 



CHAP. VII. 



EPIRRHEOLOGY, BOTANICAL GEOGRAPHY, FOSSIL BOTANY. 



EFIRRHEOLOCY (298.). DIRECTION OF ROOTS AND STEMS 



(299.) BOTANICAL GEOGRAPHY (302.). FOSSIL BOTANY. 



(315.). 



(298.) Epir rheology. THIS term has recently been 

 proposed, to express thsft branch of our science which 

 treats of the effects produced by external agents upon 

 the living plant. It can only be considered as a sub- 

 ordinate department of vegetable physiology, and one 

 indeed whose limits are not very strictly defined. For 

 we have seen that life itself requires the stimulus of 

 external agency, in order that its powers may be eli- 

 cited, and produce the various phenomena of vege- 

 tation included under one or other of the two functions 

 of nutrition and reproduction. But then these func- 

 tions become variously modified, according as the ex- 

 ternal stimuli by which they are called into action are 



