164 Darwin, and after Darwin. 



so far at all events as the higher animals are concerned, you 

 have " psychological selection." In order to give you a speci- 

 men of what can safely be done by calculation if you take 

 a problem of sufficient definiteness, I have chosen the case of 

 a flowering plant in which a certain proportion of the race 

 have developed the peculiarity of being sterile with the re- 

 mainder, while retaining the normal fertility of the race in 

 unions among themselves. In order to give the greatest ad- 

 vantage to your critics, I have assumed that such flowers as 

 possess the peculiarity are not self-fertilizable ; for it is clear that 

 if we suppose that they are self-fertilizable, the fertility need 

 be veiy slightly affected. 



As I have excluded self-fertilization, it is necessary, if we are 

 to get any trustworthy results, that one should consider the 

 mode in which fertilization will be produced. I have taken 

 the case of fertilization by insects, and have assumed that each 

 flower is visited a certain number of times by insects during 

 the period when fertilization is possible ; and, further, that the 

 insects which visit it have on the average visited a certain 

 number of flowers of the same species before they came there. 

 Of course nothing but observation can fix these latter numbers ; 

 but I should not be surprised at finding that they are of 

 considerable magnitude ^. In order to make the results a little 



' In this anticipation Mr. Moulton is right. The well-known botanist, 

 Mr. Bennett, read a most interesting paper on the subject before the 

 British Association in 1881. His results have since been corroborated 

 by other observers. In particular, Mr. R. M. Christy has recorded the 

 movements of 76 insects while visiting at least 2,400 flowers. {Entomo- 

 logist, July 1883, and Zool. Journal Lin. Soc, August 1883.) The 

 following is an analysis of his results. In the case of butterflies, in 

 twelve observations on nearly as many species, there are recorded 

 altogether 99 visits to fifteen species of flowers ; and of these 99 visits 94 

 were constant to the same species, leaving only 5 visits to any other, 

 or second species. In the case of the hive-bee, there were 8 individuals 

 observed : these visited altogether 258 flowers, and all the visits paid by 

 the same individual were paid to the same species in each of the eight 

 cases. Lastly, as regards humble-bees, there were altogether observed 

 55 individuals belonging to four species. These paid altogether 1751 

 visits to 94 species of flowers. Of these 1751 visits, 1605 were paid to 

 one species, 131 to two species, 16 to three, 6 to four, and i to five. 



