THE HABITAT FLOWERING PERIOD INSECT VISITORS. Xlll. 



ceivably be due to the drift, after passing Lanarkshire, having left along 

 the left bank of the Annan Valley and the Beeftub considerable 

 quantities of limestone fragments ; but I am not sure if this explanation 

 is correct. (See Geology). The present condition of the study of 

 botanical environment is so chaotic and speculative that it is not 

 advisable to point out more than these obvious factors. 



FLOWERING PERIOD. 



My data for this are chiefly due to the excellent observations made 

 for me by Messrs Johnstone, G. Bell, and J. Shaw. These are only 

 intended to be approximate, and in cases where the two former have 

 observed the same plant there is sometimes a difference of six weeks, 

 which is quite what one would expect. I did not realise at first the 

 importance of this question, both as proving that a definite " thermal 

 constant " is required before any given flower appears, and also in 

 practically isolating two varieties of the same species which grow under 

 different conditions of exposure. Observations are much required on 

 all these points, and both the hour of the day at which flowers open 

 and close, as well as the commencement and end of the flowering 

 season, must sometimes contribute to complete isolation of incipient 

 varieties and subspecies. 



INSECT VISITORS. 



The catching of insect visitors has probably occupied more than 

 nine-tenths of the time which I have spent on the work. I soon found 

 that no human being could expect to go through the whole Flora, 

 species by species, as it would scarcely be possible to do six species 

 thoroughly in one season, while the number here alluded to is nearly 

 900. I have therefore attempted to give an idea of the more common 

 visitors of about 270 species. Miss Hannay, Mr Armstrong, and Miss 

 Ethel Taylor have given me most valuable assistance, and a very 

 important paper by Mr J. C. Willis* has greatly pleased me, because 

 my own observations are very markedly supported by his, which were 

 taken independently at Auchencairn. 



The result is to leave me more impressed by the importance of this 

 study, for it is obvious that for the distribution of many plants, certain 

 insect visitors are absolutely essential. They are also to my mind 

 important agents in the isolation of varieties, for though the same plant 

 may grow inside a wood and in the neighbouring field, it is quite certain 

 that the same insects will not visit its flowers in each case, and hence 

 these plants are absolutely isolated. 



* Annals of Botany, vol. ix., June, 1895, 



