208 A NATURALIST IN MID-AFRICA. 



depends upon the commencement and duration of 

 the rains and on temperature. 



After our English flowers have received a definite 

 amount of sunlight (called by Korner the thermal 

 constant *) and moisture (which is rarely lacking 

 in England) they bloom.; and perhaps as much as 

 thirty days may separate the period of flowering 

 in an abnormally cold and an abnormally hot 

 year. 



Now imagine some kind of plant which is 

 travelling from the home of its parents ; it is easy 

 to see that if it enters a physically different climate 

 it will not bloom at the same period as its parents. 

 The moisture and amount of heat will be different. 



Most flowers remain in bloom for a compara- 

 tively short time. 80, assuming that the wanderer 

 does not blossom till the 1st of May, and remains 

 in flower one month, it is obvious that if its 

 parent begins flowering on the 1st of April, even if 

 insects or wind can pass freely from one to the 

 other, no cross fertilisation can take place. The 

 two are separated as widely as if the broad Atlantic 

 rolled between them. 



If there are different physical conditions, these 

 will affect the main body of a species entering the 

 country. The extreme conservatives and over- 

 rash radicals, who either refuse to alter their 

 habits or alter them too thoroughly, will be ex- 

 ceptional and will leave no descendants. 



* See " Natural History of Plants," Korner and Oliver. 



