THE PHENOMENA OF MUTATION 193 



to swim with its eyes half in and half out of water. 

 Are we to suppose that the upper half of the body or 

 eye had a positive heliotropism and the lower half a 

 negative heliotropism ? The fact is that the fish 

 swims at the surface in order to watch for and feed 

 on floating particles. The tropism concerned is the 

 food tropism, but what is gained by calling the 

 search for food common to all active animals a 

 tropism, and how is the search for food before the 

 food is perceptible to the senses, before it can act as 

 a stimulus on a food-sensitive substance in the body, 

 to be compared to a tropism at all ? 



Loeb undertakes to prove that the organism as 

 a whole acts automatically according to physico- 

 chemical laws. But he misses the question of 

 evolution altogether. For example, he quotes 

 Gudernatsch as having proved that legs can be 

 induced to grow in tadpoles at any time, even in 

 very young specimens, by feeding them with thyroid 

 gland. Loeb writes : ' The earlier writers explained 

 the growth of the legs in the tadpole as a case of 

 an adaptation to life on land. We know through 

 Gudernatsch that the growth of the legs can be 

 produced at any time by feeding the animal with the 

 thyroid gland.' Obviously he thinks that these 

 two propositions are contradictory to each other, 

 whereas there is no contradiction between them at 

 all. Loeb actually supposes that the thyroid is the 

 cause of the development of the legs. Logically, if 

 this were the case it would follow that if we fed an 

 eel or a snake with thyroid it would develop legs like 

 those of a frog, and if a man were injected with 

 extract of the testes of a stag he would develop 

 antlers on his forehead. It will be obvious to most 



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