THE FOOD OF YOUNG ANIMALS 203 



on the other hand, select their prey by sight, and they quickly 

 starve if from any accident they become blind. 



A set of very famous experiments was made many years ago on 

 the feeding of the tadpoles of the common frog. Young tadpoles 

 were believed not to have quite decided as to whether they were 

 going to become males or females, and their anatomy shows that 

 they retain at least a good many of the structures of both sexes 

 until they are nearly ready to go through the metamorphosis, 

 during which the organs of one sex develop and those of the other 

 degenerate. In an ordinary set of young tadpoles nearly ready 

 to become frogs, the sexes are fairly evenly balanced, but there is 

 a slight excess of females. E. Yung, a distinguished French 

 naturalist, fed one set of very young tadpoles entirely on beef until 

 they were nearly mature, a second set on fish and a third set on 

 the flesh of frogs. He found that in the first set the percentage 

 of females rose to 78, in the second to 81 and in the third to 92. 

 It seemed, in fact, as if the food most like the body of the adult, 

 and therefore most nutritious for the animal, favoured the produc- 

 tion of females, whilst a poorer fare led to the production of males. 

 Other observers have repeated the experiments, but with conflicting 

 results. I, myself, reared tadpoles for several successive summers, 

 feeding one set of a hundred on vegetable matter with very little 

 animal food, and another set entirely on animal food, and examining 

 the sexes afterwards. I got results that differed widely from year 

 to year. The chief trouble is that however carefully the tadpoles 

 be kept, with plenty of running water and with the removal of all 

 fragments of food soon after each meal, the mortality is very high, 

 and it is a fortunate result to rear twenty or thirty out of the hundred. 

 Similar attempts have been made to decide the sex of higher animals, 

 even of human beings, by the kind of food given to the mother 

 before the young are bom, but there has been no success, and most 

 naturalists now believe that, at least in the vertebrate animals, the 

 sex is not determined by external conditions such as the nature of 

 the food. 



