cH. XIX Influence of Habits and Surroundings 305 



changes produced in the body by use or disuse can be 

 transmitted to the offspring, (3) that the theory of the 

 accumulation of (unexplained) favourable variations in the 

 course of natural selection seems logically sufficient. I 

 should suspend judgment, because it is unprofitable to argue 

 when ascertained facts are few. 



But if you like to argue about probabilities, the following 

 considerations may be suggestive : — 



The natural powers of animals — horses, dogs, birds, and 

 others — can be improved by training and education, and 

 animals can be taught tricks more or less new to them, but 

 we have no precise information as to any changes of 

 structure associated with these acquirements. 



Individual animals are sometimes demonstrably affected 

 by use or disuse. Thus Packard cites a few cases in which 

 some animals — usually with normal eyes — have had these 

 affected by disuse and darkness ; he instances the variations 

 in the eyes of a Myriapod and an Insect living in partial 

 daylight near the entrance of caves, the change in the eyes 

 of the common Crustacean Gammarus pulex after confine- 

 ment in darkness, the fact that the eyes of some other 

 Crustaceans in a lake were smaller the deeper the habitat. 



There are many more or less blind animals, and Packard 

 says " no animal or series of generations of animals, wholly 

 or in part, can lose the organs of vision unless there is some 

 appreciable physical cause for it." If so, it is probable that 

 the appreciable physical cause has been a direct factor in 

 producing the blindness. 



Not a few young animals have structures, such as eyes 

 and legs, which are not used and soon disappear in adult 

 life. Thus the little crab Pt?z?totheres, which lives inside 

 bivalves and sea- cucumbers, keeps its eyes until it has 

 established itself within its host. Then they are completely 

 covered over and degenerate. The same is true of many 

 internal parasites, and Semper concludes that " we must 

 refer the loss of sight to disuse of the organ." Perhaps the 

 same is true of some blind cave-animals, in which the eyes 

 are less degenerate in the young, and of the mole, whose 

 embryos have between the eyes and the brain normal optic 



X 



