EVOLUTION IN NATURE 155 



gradually to a condition of independence from the 

 fluctuations of the environment. And as an im- 

 portant point, they show that the species and sub- 

 specific units of our present classification are not 

 always based on characters which are hereditary 

 in the usual sense, but may sometimes be produced 

 in part by specific conditions of the external en- 

 vironment. 



When we seek evidence concerning the possible 

 inheritance of some effect of individual response to 

 environment we find that it is meager, although 

 many experiments have been conducted and a 

 few have given positive results of some degree. 

 The experiments of Guyer and Smith on cytolysins 

 in the production of eye defects in rabbits are too 

 well known to need detailed mention. They are 

 interesting, and in spite of the skepticism of some 

 writers it seems to me that they have an important 

 significance, but they appeal to me much less than 

 experiments which have to do with responses to 

 conditions which might very well occur in a nat- 

 ural environment. Among the latter a number 

 deserve consideration. 



Woltereck subjected lots of Daphnia to over- 

 feeding, and noted as a result the development of 

 higher heads. ^^ When the animals were returned 

 to normal surroundings after the first appearance 

 of the modification their offspring were normal. 



" Verh. d. Deutsch. ZooL Gesell, 1909, pp. 110-172.^ ^ 



