EFFECTS OF ENVIRONAL PRESSURE 217 



hindering the use of an organ, or a faculty, may 

 strengthen or weaken it : the resulting develop- 

 ment or degeneration may become fixed heredi- 

 tarily in the offspring. The eyes of a newt 

 (Proteus), which lives in the dark waters of the 

 Adelburg cave, have become quite rudimentary 

 and sightless. The horses of a mountain country 

 are born with hoofs that will stand the jarring 

 of hard rocks and city pavements much better 

 than those of lowland breeds. Physical changes 

 that are caused by mutilation are, as a general 

 rule, not inherited ; nor should we expect them 

 to be, since sudden modifications of an organ would 

 not affect the germ-plasm so strongly as modi- 

 fications which have resulted from gradual pres- 

 sure. But cases are numerous in which mutila- 

 tions have been passed on to offspring : in one 

 of the most curious, a cat with a broken tail 

 bore kink-tailed kittens, her tendency to transmit 

 her defect increasing with each successive litter 

 until not one of her kittens was born with a 

 straight tail. It has been demonstrated that 

 guinea pigs, which have become epileptic from 

 a surgical operation, may transmit an epileptic 

 tendency to their offspring. 



A sudden change of environment appears to 

 stimulate the occurrence of mutations or "sports." 

 Under the artificial conditions of domestication 

 plants and animals sport much more freely than 

 in their natural wild state display, that is to say, 

 eccentricities of form which are heritable and are 

 seized upon by breeders as a means of developing 

 new varieties. The numerous breeds of dogs, 

 fowls and pigeons have for the most part sprung 

 from these sudden eccentricities ; to sports, 

 assisted by cross breeding and intensive cultiva- 

 tion, we owe a large proportion of our most 

 conspicuous garden flowers and our daintiest 



