THE ADAPTATION OF ORGANISMS 69 



course of the summer, when food is abundant, multiply 

 very rapidly, and that all the offspring are females, and 

 that only when food becomes scarcer and conditions less 

 favourable does ordinary sexual reproduction recur. 

 Yung's classical experiments on the feeding of tadpoles 

 are often brought forward as another proof of this. In 

 these experiments he thought he had proved that the 

 alteration of the food was responsible for the large 

 increase, from about 57 per cent., which is the normal, 

 to 92 per cent, of females, and consequently the corre- 

 sponding decrease in the percentage of the males. One 

 point unfortunately was neglected in these experiments, 

 and this point is fatal as far as the acceptability of the 

 results are concerned he forgot to take into considera- 

 tion the sex of the individuals that died during the 

 course of the experiments. 



Of the physical forces that affect the organism, we 

 need only consider light and heat. A rise in tem- 

 perature is usually accompanied by a rapid increase in 

 the rate of multiplication, as Maupas has shown in his 

 experiments on Stylonichia. A decrease in warmth has 

 generally the reverse action ; it diminishes the rate of de- 

 velopment and often tends to produce dwarfed or larval 

 forms, while the cold of winter does have some con- 

 siderable share in the production of the winter coat of 

 many diverse forms of animals. 



Light also is important, but it is not by any means 

 easy to give a general explanation of its influence. We 

 know that it has considerable influence in the formation 

 of chlorophyll, and it is just possible that it may have a 

 direct influence on the formation of pigment in various 



