312 The Study of Animal Life PART iv 



stomach of gulls and other birds may be experimentally 

 altered by change of diet, and the same is seen in nature 

 when the Shetland gull changes from its summer diet of 

 grain to its winter diet of fish. The colours of birds' feathers, 

 as in canaries and parrots, are affected by their food. A 

 slight difference in the quantity and quality of food deter- 

 mines whether a bee -grub is to become a queen or a 

 worker, royal diet evolving the reproductive queen, sparser 

 less rich diet evolving the more active but unfertile worker. 

 Abundant food favours the production of female offspring, 

 while sparser food tends to develop males. Thus, in frogs, 

 the proportion of the sexes is normally not very far from 

 equal ; in three lots of tadpoles an average of 5 7 per hun- 

 dred became females, 43 males. But Yung has shown that 

 the nutrition of the tadpoles has a remarkable influence on 

 the sex of the adults. In a set of which one half kept in 

 natural conditions developed into 54 females to 46 males, 

 the other half fed with beef had 78 females to 22 males. 

 In a second set of which one half left to themselves 

 developed 61 females to 39 males, the other half, fed with 

 fish, had 8 1 females to 19 males. Finally, in a third set, 

 of which one half in natural conditions developed 56 females 

 to 44 males, the other half, to which the especially nutritious 

 flesh of frogs was supplied, had no less than 92 females 

 to 8 males. 



When food is abundant, assimilation active, and income 

 above expenditure, the animal grows, and at the limit of 

 growth in lower animals asexual multiplication occurs. 

 Checked nutrition, on the other hand, favours the higher 

 or sexual mode of multiplication. Thus the gardener 

 prunes the roots of a plant to get better flowers or repro- 

 ductive leaves. The plant-lice or Aphides, which infest 

 our pear-trees and rose-bushes, well illustrate the combined 

 influence of food and warmth. All through the summer, 

 when food is abundant and the warmth pleasant, the 

 Aphides enjoy prosperity, and multiply rapidly. For an 

 Aphis may bring forth young every few hours for days 

 together, so rapidly that if all the offspring of a mother 

 Aphis survived, and multiplied as she did, there would 



