BIRDS AND EVOLUTION 377 



modifiability ; but we do not know of any convincing 

 evidence to show that this repercusses on the race. 



The colours of birds' feathers may be altered notably 

 by a change in their food, as in the case of canaries and 

 parrots. Mr. C. W. Beebe has shown that some birds, 

 such as the Bobolink, may be dieted so that they keep 

 their breeding plumage throughout the year and may sing 

 their spring song in mid-winter. This is a remarkable 

 modification. 



Modifications are common among birds on a small scale 

 at least, and they may affect rather subtle qualities such as 

 fecundity (the egg-laying of poultry), fattening, and power 

 of flight. But while they may be of much individual im- 

 portance, it has not been proved that they directly affect the 

 race. In other words, we do not know that they are trans- 

 missible. The ancestral jungle-fowl will lay at the most 

 40-50 eggs in its lifetime, while a domestic hen of superlative 

 egg-laying quality may lay 3000 ! This is partly due to 

 stimulating nutrition producing its effect afresh on successive 

 generations of tfidtviduals, and partly to the selection of 

 strains of high fertility. 



It is easy enough to interpret a peculiarity in a bird as 

 due to the cumulative transmission of bodily modifications 

 (or individually acquired characters), but it does not follow 

 that the peculiarity in question arose in that way. The 

 burrowing parrot, Stringops, has ceased to fly, and the keel 

 on its breastbone is practically absent. It is easy to interpret 

 this as due to degeneration following disuse of the muscles 

 of flight, but what actually happened may have been very 

 different. It may be that the ancestors of the present-day 

 Stringops suffered from a constitutional variation in the 

 direction of weakened muscles and dwindling keel, and that 

 they therefore took to burrowing . This is one of the con- 

 tinually recurrent dilemmas of aetiology, and the only 

 solution is along the lines of open-mindedness and active 

 scepticism. 



It has often been said that the hereditary occurrence of 

 callosities on appropriate places of an animal's body points 



