198 INTRODUCTION 
From this table it appears, for example, that Class E attracts allotropous 
Diptera more than the other classes. The proportion (20 %) of visits of Diptera to 
this class of flowers is higher than the proportion (8-6 %) of visits of insects in 
general, from which it may be concluded that Class E possesses means of allurement 
by which allotropous flies are specially attracted. By comparing the table relating to 
a particular insect group with that for insects in general, it is therefore possible 
to determine—for a given month and district—the flower classes which the insect 
group in question prefers or avoids. 
MacLeod has worked through on this method Miiller’s observations in the Alps 
and Loew’s observations in the botanic gardens at Berlin, and has drawn up ten 
different series according to the month and place of observation, so that the choice 
of flower of each insect group could be determined ten times. 
The clearness of MacLeod’s method is enhanced by the fact that he gave 
a graphic representation of his results. For each month he erected upon a 
horizontal line—at equal distances from one another—seven ordinates roomm. 
long, corresponding to the flower classes, and then measured off on each ordinate 
a length representing the proportion in which the class in question was visited by 
insects in general. By connecting these points with one another, he obtained a 
broken line which he described as the general insect line. In similar fashion he then 
drew special lines for the individual insect groups, so that the choice of flowers made 
by the several groups could be seen at a glance. Wherever a special line runs 
above the general insect line, the corresponding insects must have a preference for 
the particular flower class, and, conversely, the deeper a special line sinks below the 
general insect line, the greater is the repugnance manifested by the particular insect 
group towards the flower class indicated. To be of any value this graphic method 
must give cons/ant results, and must therefore yield she same result for the same 
insect group and the same flower class, in each of the ten series of observations 
collated by MacLeod. Such uniformity obtains for the following cases :— 
1. Beetles show a constant preference for An and E, while C, H, and L are the 
most repugnant to them. 
2. Allotropous Diptera prefer E, and always reject H. 
. Hemitropous Dipiera consistently prefer EC, and reject H. 
. Short-tongued Bees always avoid H. 
. Long-tongued Bees avoid E and S, and consistently prefer H. 
. Lepidoptera consistently prefer L, and reject E. 
Non > WwW 
Although in other cases there was no agreement between the ten series of 
observations collated by MacLeod, it was demonstrated that the groups of flowers 
and insects are not homogeneous. In those cases where, on Muller’s theory, 
strong preference or the opposite could be inferred of certain visitors in regard to 
certain flower classes, reasoning from the structure of the insects and of the flowers 
they visit, MacLeod’s results were constant. 
By this graphic method relations can be recognized which, although theoreti- 
cally probable, were not deduced by the older ways of dealing with statistics, e.g. 
the preference of Lepidoptera for lepidopterid flowers comes out very clearly. 
MacLeod’s method would furnish still more reliable results by taking smaller 
