40 HEREDITY 



fishes. Fishes are found in numerous deep caves, where 

 there is absolutely no light. It is often possible to tell, 

 from geological indications, about how many thousands 

 or tens of thousands of years any particular cave has 

 existed. The eyes of these fishes are invariably more 

 or less reduced, in some cases being nothing more than 

 mere rudimentary and useless specks. It is assumed, 

 of course, that the fishes had normal eyes at the time 

 of their imprisonment. It is found that the degree of 

 degeneration of the eyes always corresponds pretty 

 closely with the length of time that the fishes have been 

 in the cave. The process of losing the eyes seems to be 

 extremely gradual and slow. The inheritance of the 

 effects of disuse is the obvious explanation, and any 

 other must appear both less simple and less probable. 



Actual experimental results bearing on this question 

 are very difficult to obtain. Use or disuse is supposed 

 to have only a very slight hereditary effect, so that 

 several generations would have to elapse before this 

 was noticeable. Hence it is not negative proof to say 

 that the parson's son is as strong of arm as the black- 

 smith's, or the cobbler's as upright as the sergeant- 

 major's. All that we can say from such ordinary 

 observation is that the effects of use and disuse, if 

 they are inherited at all, must be so to a very slight 

 extent. And it would seem almost impossible to devise 

 an experiment to prove more than this. 



But the slightest of effects, if cumulative through 

 many generations, would ultimately bring about results 

 of tremendous importance. So that if the inheritance 

 of the effects of use and disuse be ever so slight, it may 

 still be ever so important. 



Another group of acquired characters may be de- 

 scribed as the direct effects of environment. Changed 

 conditions sometimes produce an effect on the organism 

 which is not, so far as we can see, adaptive ; in other 

 words, the change brought about in the organism does 

 not appear to render it better suited to the new con- 

 ditions. For instance, the colours of the wings of 

 certain butterflies vary according to the temperature 



