ACTION OF FORCES ON ORGANISMS. 4S 



size and general development those which grew 

 in their natural surroundings. Dr. Born has shown 

 that in their natural conditions of development the 

 number of males among the young is equal to the 

 number of females. 



Professor Yung's experiments were carried on pri- 

 marily to determine the effect of different foods on 

 the general development of the tadpoles. He had six 

 glass jars arranged, so that all the conditions of light, 

 temperature, etc., should be as much the same as 

 possible, and so that the only differences in the six 

 jars should be the different food placed in each one. 

 At the beginning of the experiment, on the first of 

 April, fifty specimens in the earliest tadpole stage 

 were placed in each jar. The mortality was very 

 great, and on the thirtieth of June, there remained of 

 the three hundred tadpoles, seventy-four which had 

 completed their metamorphosis into frogs. In two 

 of the jars, all died without metamorphosis, and in 

 the other four, the proportion of females ranged from 

 seventy to seventy-five per cent. Professor Yung's 

 general results were as follows : " i. That tadpoles of 

 the frog, taken from the same batch of eggs, develop 

 in a very different manner, according to the kind of 

 food which they receive. 2. The different kinds 

 of food in question, taken as a single article of diet, 

 favour development in the following order, — flesh of 



