392 THE STUDY OF ANIMAL LIFE CHAP. 



stomach of gulls and other birds may be experimentally 

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

 when the Herring 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. Mr. Beebe, Curator of birds in the New York 

 Zoological Gardens, has shown that some birds, such as 

 the bobolink, may be dieted so that they keep their 

 breeding plumage throughout the year and will sing their 

 spring song in mid- winter. This is biological magic. 



Very striking, too, are Prof. Child's experiments on 

 the effect of altered diet on Planarian worms. Thus 

 a diet of freshwater mussels, for some unknown reason, 

 depresses the vitality, i.e. lessens the rate of metabolism 

 and the power of resistance. The stock becomes senescent, 

 and if the diet be continued for several generations there 

 is an aggravation of senescence, for the individuals begin to 

 be "born old." The effect of the mussel diet is cumulative. 



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 reproductive 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 in the course of a year be a progeny 

 which would weigh doAvn 500,000,000 stout men. But 

 all through the summer these Aphides are wholly female, 

 and therefore wholly parthenogenetic ; no males occur. 

 In autumn, however, when hard times set in, when food 

 is scarcer, and the weather colder, males are born, par- 

 thenogenesis ceases, ordinary sexual reproduction recurs. 

 Moreover, if the Aphides be kept in the artificial summer 



