144 THE CAUSATION OF DISEASE. 



interest. Now, in all cases in which the organism is educated to 

 a rhythm, the effect is produced by a rhythm in the external E 

 impressing a corresponding rhythm upon certain tissues. 



(c) Use and Disuse of Parts. — The former, we know, causes 

 hypertrophy of a part, the latter atrophy or dwindling of 

 it. The result is due in both cases to an alteration of E. 

 Let us instance the case of muscle. The E of each individual 

 muscle-fibre is widely different when a muscle is constantly 

 used to what it is when kept in constant disuse. This fact 

 is so obvious to the physiologist that it is unnecessary to 

 specify the difference in the two cases. Now, this modification 

 in muscle-cell-E is in response to a modification in the ex-cor- 

 poreal-E, and therefore the effect upon the muscle is indirectly 

 brought about by the external-E of the individual. If an 

 animal ceases to have need of a part, it is because the external 

 conditions have altered. 



The drooping of the ears in domesticated animals, for in- 

 stance, is manifestly due to an alteration in their external condi- 

 tions, and this latter is also the cause of the structural alterations 

 in such cases as the following : — " In the domesticated duck,* 

 the bones of the wing weigh less, and the bones of the leg more, 

 in proportion to the whole skeleton, than do the same bones in 

 the wild duck ; and this change may be safely attributed to the 

 domestic duck flying much less, and walking more, than its wild 

 parents. The great and inherited development of the udders 

 in cows and goats in countries where they are habitually 

 milked, in comparison with the state of these organs in other 

 countries, is probably another instance of the effect of use. 

 Not one of our domestic animals can be named which has not 

 in some countries drooping ears ; and the view which has been 

 suggested that the drooping is due to the disuse of the muscles 

 of the ears from the animals being seldom much alarmed seems 

 probable." 



(d) Correlation. — According to this principle, alteration in one 

 part is apt to go hand in hand with alteration in some other part 

 or parts. Thus, white torn cats have blue eyes ; hairless dogs, 

 imperfect teeth ; pigeons with short beaks have small feet, and 



* " Origin of Species," p. 8, sixth edition. 



