METHODS OF RESEARCH IN FLOWER POLLINATION 197 
To eliminate the zwfluence of the Composition of the Flora upon the choice made 
by insects, a standard must be found for the proportions in which the various flower 
classes are represented in the flora of each month. The mere numbers of species 
belonging to each class that are in flower would not be sufficient, for in this way 
small, inconspicuous blossoms which attract but few insects would have the same 
value given them as large, conspicuous forms, rich in nectar, and attracting many 
insects. The actual standard indicating the importance of each class seems rather 
to be the degree in which the seven flower classes are visited by a// insects. The 
number of visits that each class of flower received depends upon (a) the number of 
flowers in bloom, (4) the number of individuals (frequency or rarity of species), 
(c) the abundance of nectar, (¢) the size and colouring of the conspicuous parts, 
i.e. the totality of the means of attraction, che physiognomic value of each class in the 
flower world. 
MacLeod estimates for the species of each individual flower class the per- 
centages of insect visits observed during a given month, and thus reaches approxi- 
mately the degree to which the flower class in question attracts insects as a whole, 
each group of insects being separately considered. If, for example (according to 
Hermann Miiller’s observations in the Alps), during the month of June, all the 
flowers in the Alps below the limit of trees receive 947 different visits, these— 
according to MacLeod’s calculation—would be distributed as follows among the 
flower classes :— 
Viszts. Per Cent. 
Class An 25 2:6 
eb SERRE 8.6 
barr 2g0 28-5 
eh dhe ye SOM 144 
” S 146 15°4 
ie, uke VOR 21-1 
ean 847 9:2 
Total 947 99:8 
Of the 947 visits, 201 would be paid by allotropous Diptera (i.e. all the antho- 
philous flies except Syrphidae, Bombyliidae, and Conopidae). If these were equally 
attracted by the seven flower classes, the 201 visits would be divided between the 
classes in the same proportions as the 947 visits of insects collectively were divided. 
But—as the following table shows—the proportions are quite otherwise: the visits of 
allotropous Diptera are paid as follows :— 
Visits. Per Cent. 
Class An 3 1-5 
Mahi 40 20:0 
i, Oe 41:0 
aa 45 22:5 
2 Oe 13°5 
a fe) 0-0 
Maas 4 2-0 
Total 2o1 1005 
