The Mason-bees 



I have reverted to this case to show the 

 difficulties that beset the mian who confines 

 himself to casual observations, however care- 

 fully carried out. One should never rely upon 

 a lucky chance, which may not occur again. 

 We must multiply our observations, check 

 them one with the other; we must create inci- 

 dents, looking into preceding ones, finding out 

 succeeding ones and working out the relation 

 between them all : then and not till then, with 

 extreme caution, are we entitled to express a 

 few views worthy of credence. Nowhere do 

 I find data collected under such conditions; for 

 which reason, however much I might wish it, It 

 is impossible for me to bring the evidence of 

 others in support of the few conclusions which 

 I myself have formed. 



My Mason-bees, with their nests hanging 

 on the walls of the arch which I have men- 

 tioned, lent themselves to continuous experi- 

 ment better than any other Hymenopteron. I 

 had them there, at my house, under my eyes, 

 at all hours of the day, as long as I wished. 

 I was free to follow their actions in full detail 

 and to carry out successfully any experiment, 

 however long. Moreover, their numbers al- 

 lowed me to repeat my attempts until I was 

 1^4 



