METHODS OF RESEARCH IN FLOWER POLLINATION 195 



species of flowers by biting them tlirough. Bombus terrester also frequently plays 

 the part of a nectar-thief (cf. pp. 1 16-17). 



As already mentioned (see p. 66) Loew has proposed a classification of 

 flowers corresponding to the allotropous, hemitropous, and eutropous groups 

 of insects. Under Allotropous Flowers he places the classes An, Po, E, and EC 

 under Hemitropous Floivers the classes C and S and under Eutropous Flowers 

 the bee flowers (Hb), humble-bee flowers (Hh), and lepidopterid flowers (L). 

 The statistical investigations in flower pollination made by Loew, as well as by 

 MacLeod, Heinsius, and myself, show that there actually is a marked agreement 

 between the corresponding groups of flowers and insects. But there is still a need 

 of more numerous and more thorough special investigations to make quite clear the 

 relations between the two sets of groups. 



The four groups of flower guests established by Loew are connected with one 

 another by intermediate stages and transitional forms. This classification leaves 

 untouched those theoretical speculations which have reference to the genetic develop- 

 ment of the various insects. 



X. Methods of Research in Flower Pollination. 



The statistical method introduced by Hermann Miiller to determine the 

 reciprocal dependence between flowers and insects, and more especially employed 

 by him in his 'Alpenblumen' (pp. 477-525), does not count the individual visits of 

 insects to a species of flower, but only the number of insect species which seek out 

 a particular kind of plant. At first sight this method seems unreliable, if not quite 

 worthless. As a matter of fact it cannot be denied that a disadvantage is involved, 

 though it is one that can hardly be avoided, for it is almost impossible to count all 

 the individual visits that a conspicuous flower receives during a considerable period 

 of observation when the weather is fine, or to establish how many flowers are 

 pollinated by a species of insect in a given time^. 



The value of a method must, however, be judged by its results. Those 

 attained by this method, as employed first by Hermann Miiller and afterwards by 

 E. Loew, J. MacLeod, and myself, show that (to quote from Loew, ' Blumenbesuch,* 

 II, p. 147) it has greater possibilities than any one possessing only a superficial 

 knowledge of it would be inclined to believe. In particular the reproach that it must 

 aff'ord an inaccurate idea of the number of pollinators, because it counts the visits of 

 species and not of individuals, is of no importance. This appears from the agree- 

 ment which exists between the determinations of Miiller and those of all subsequent 

 observers regarding the numerical relation of visits not the absolute values, of 

 course, but their proportions in a series and many of the sets of observations were 

 made quite independently of one another. 



' Professor F. Dahl, of Kiel, informs me that he has constructed an apparatus that automatically 

 catches all the visitors of a flower or an inflorescence. This would make it possible not only to 

 ascertain the number of insect species that visit flowers, but also the number of individuals visiting a 

 flower within a given time. Meanwhile Prof. Dahl will make a communication with regard to this 

 apparatus at the meeting of the Deutsche Zoologische Gesellschaft (1898) at Heidelberg. The trap 

 described and figured by him in his memoir ( ' Vergleichende Untersuchungen liber die Lebensweise 

 wirbelloser Aasfresser,' SitzBer. Ak. Wiss., Berlin, 1896) is made on a similar principle. 



O 2 



