90 HEREDITARY CHARACTERS 



seems more reasonable to explain the one exceptional case 

 in the light of what usually happens, than to seek a special 

 explanation, contrary to what is usual, for the one exception, 

 particularly when there is a simple explanation perfectly 

 compatible with what occurs in all. 



My friend Professor Harvey Gibson has described to me a 

 case of gradual adaptation which came under his own obser- 

 vation. When he was a student of the late Professor Dixon 

 in Edinburgh, it was observed that the flowers in a bed of 

 Campanulas failed to set seed. He was deputed to find out 

 the cause. He found that the humble bees had taken to 

 piercing the calyx of the flower, and extracting the honey 

 thus instead of in the usual way, which would have ensured 

 pollination. A few plants, however, set seed, and from these 

 new plants were grown. In the next generation more flowers 

 set seed, and still more in the third. Upon microscopical 

 examination it was found that a layer of sclerotic tissue was 

 generally developed in the calyces of the flowers in this 

 generation, so that it was more difficult or impossible for the 

 bees to pierce them. The bees were therefore forced to 

 pollinate the flowers which had developed the most sclerosis 

 if they wished to get at the honey. This appears easily 

 explicable upon the selection of small variations. There is 

 no evidence whatever of a mutation. 



In all the examples cited there is a strong suggestion, in 

 some of them there is a great deal of direct evidence, that 

 the changes have been brought about in a gradual manner 

 by the accumulation of small variations. We must appa- 

 rently admit, then, that adaptation is generally produced 

 by a series of very small variations, but there must also be 

 some means by which the adaptations are kept up to the 

 required standard and become even more accurately suited 

 to varying and to more stringent conditions. Any theory which 

 professes to explain how new characters arise, must explain 

 this also. As small variations are always occurring round the 

 mean of every character of a race, the action of the surround- 

 ing conditions must be, that those individuals best adapted 



