HISTORICAL. 133 



the white clover, so in like manner a monotropic bee visits but a single kind of flower. 

 But in the former case the bee flies throughout the whole season, while in the latter 

 when the flower fades the bee's period of flight is over." 



Kranichf eld's observations. — Kranichfeld (1915:39) has carried out 

 two series of field observations on the constancy of the honey-bee to color 

 and species. Of the 18 fields studied, 15 were being visited by honeybees 

 and 10 of these contained plants of Cirsium oleraceum. In these Cirsium 

 was sought exclusively in five cases and chiefly in all the others, the only 

 visits to other flowers being to Lathyrus in one case, Cirsium palustre in 

 another, and to Centaurea and Heracleum in the third. As the flowers were 

 almost wholly white or dull in color, it was concluded that color was not 

 decisive in the choice of the flowers. In the second series the constancy 

 was greater, both as to color and species, the exceptions being regarded as 

 due to the behavior of the young and inexperienced bees. The other cases 

 of inconstancy appeared to be due to the confusion of the honey-bees and 

 bumble-bees by the presence of the same color in two or three species. 

 While the results were not entirely harmonious, they strengthened the 

 probability that sense of color serves to guide bees and hence is a factor in 

 constancy. 



Origin of oligotropism. — Robertson (1914) ascribes the assumption 

 of the oligotropic habit to competition, maintaining that "the bee fauna 

 is all that the flora will support, that there is constant competition between 

 bees, and that natural selection favors those which are the least competitive 

 in food habits. The early maximum flight, the non-competitive pheno- 

 logical distribution, and the frequent oligotropic habits indicate that these 

 bees have managed to hold their own only by dividing up the remaining field 

 and occupying the most favorable corners left by their polytropic compet- 

 itors." On the contrary, Lovell (1914 2 ) believes that only a part of the 

 available flower food is gathered by bees, many plants producing nectar 

 far in excess of the needs of the tributary bee population. If severe com- 

 petition were to occur, the oligotropic habit would be undesirable, since 

 such a bee would be at a disadvantage in comparison with polytropic spe- 

 cies, unless it were always certain to find the requisite food supply. In the 

 genus Perdita, the tube-length of the flower, and not competition, is the 

 factor that limits the visits of the various species. It is concluded that 

 "certain bees have become oligotropic because of the direct advantage 

 gained, combined with the fact that their flight was S3mchronous, or nearly 

 so, with the period of inflorescence of the plant to which they restricted 

 their visits. This theory offers an explanation of the rise of oligotropism 

 by the observation of existing conditions. There may be and often are 

 accessory factors, but they are of secondary importance." It appears 

 fairly certain that Lovell is correct in assigning the chief importance to the 

 limiting action of structure and to the coincidence of flight and flowering 

 period, though there are doubtless cases in which competition enters to 

 some degree, as apparently he would admit. Special studies are greatly 

 needed in the case of each oligotropic species to determine the exact relations, 

 and these will afford further opportunity for the use of experiment. 



