X. CLIMATE OF PLANTS. 477 



conditions of species-distribution ; although it is to be 

 feai'ed that such inferences will be imperfect and un- 

 precise. Mean annual temperature was looked to almost 

 exclusively at one time, as affording the most satisfactory 

 explanation of floral distribution. Plenty of facts were 

 adduced to sujjport this mode of accounting climatally for 

 the similarities and diversities of floral distribution ; 

 closely followed by plenty of exceptional facts also. 

 Then, the climate of the different seasons was resorted 

 to, especially the contrasts of summer and winter tem- 

 peratures, TUider the idea of qualifying the indications of 

 mean yearly temperature, gradually ascertained to be too 

 vaguely general, and subject to too many exceptional 

 contradictions. The extremes of temperature and other 

 circumstances have also been taken into account, and 

 have occasionally seemed to account for peculiarities of 

 distribution. Altogether, much interesting and explana- 

 tory information on the subject has been gi-adually accu- 

 mulated ; though unfortunately it is still too vague and 

 incomplete for plenary satisfaction or practical usefulness. 

 Among the latest fancies, is that of adding together the 

 daily temperatures into a gross amount or sum-total of 

 temperature, to which plants are supposed to be subjected 

 during the period of their growth. This is in effect the 

 tempei'ature of the year, leaving out those portions of the 

 year during which the temperature is believed to be too 

 low to have anj' vivifying or developing influence on the 

 species under consideration. It cannot be denied that 

 there is a substantial basis of truth for this improvement 

 on the older estimates of mean temperature. But it will 

 be found vastly more difticult in application ; and it opens 

 the way to a " cooking of accounts " almost at will. 

 Wherever results or sums -total do not accord with pre- 

 conceived expectation, they may be corrected and brought 



