196 INTRODUCTION 
The value of this method, continues Loew, lies also in the fact that it is not 
a purely statistical one, furnishing only averages, but specifically oecological, by 
which the behaviour of individual insects with regard to flowers can be quite as well © 
established as the behaviour of a complete group. The method has afforded 
valuable proof of the general correctness of Miiller’s rule of colour-preference, 
i.e. that insects which are more perfectly adapted to pollination generally affect 
dark colours (blue, red, violet), while insects which are less specialized prefer light 
colours (white, yellow). And, lastly, it is proved that insects choose flowers in the 
series as they would be expected to from the structure and length of their suctorial 
apparatus, and the nature of their general bodily equipment, i.e. ¢hose pollinators and 
flowers which theoretically appear to be adapted to one another, are the very ones which 
actually exert the most marked mutual attraction. ‘This proposition of Loew’s 
(‘ Beitrage,’ p. 15) was previously postulated by Miiller, but was only proved by 
later statistics. 
The two following conclusions embody the general results of the statistical 
investigations made by myself in accordance with this method (‘ Bliitenbesucher,’ 
iT, ‘p..9). 
1. Zhe more specialized a flower—t.e. the more complex its structural arrange- 
ments and the more deeply seated tts nectar—the less are tts insect visttors indiscriminately 
drawn from the entire insect fauna of a district, and the more do they belong to one or 
several similar spectes adapiled to pollination. 
2. Lhe flatter and more superficial the position of the nectar, the more varied are 
the visitors in different regions, and the more are they indiscriminately drawn from the 
entire insect fauna of the region tn question. 
MacLeod (‘De bevruchting der bloemen door de insekten,’ Verhandlingen van 
het eerste Nederlandsch Natuur- en Geneeskundig Congress, gehuiden te Amsterdam, 
op 30 Sep.-1 Oct. 1887, Amsterdam, 1888) has tested Miiller’s method from a 
new point of view. This investigator (op. cit., pp. 83-90) proceeds from the idea 
that always and everywhere the degree to which any particular group of insects 
visits a particular class of flowers depends upon three factors, i.e—1. On the Chorce 
of Flowers by insects, or in other words, on the preference of insects for certain 
flowers. 2. On the Composition of the Flora, i.e. on the proportions in which the 
various classes of flowers are represented in a district. 3. On the Zime of Year, by 
which quite different species of flowers are offered to visitors during different months, 
The first of these three factors, according to MacLeod, is constant for similar 
flowers and similar insects, which must be determined by statistics. The two other 
factors are varying quantities which must be eliminated before we can attain to 
a constant result. 
The czjfluence of the Time of Fear can easily be eliminated by separating from 
one another the observations for various months and regions, and considering them 
individually. MacLeod divides the summer half-year (in central Europe from the 
first of April to the first of October) into intervals of thirty days, and places the 
observations in so many series according to their date. Each result is therefore 
repeated as often as there are monthly series, so that the degree of reliability of each 
conclusion can be estimated. It may be assumed that the plants in flower during 
a particular period of thirty days remain much the same. 
