ACQUIRED CHARACTERS xli 



in a similar manner, owing to pursuit by man. All these 

 creatures are liable to fits of anger, in whicb they desire to 

 fight ; but, since neither their legs nor their mouths are 

 suitable for use as weapons, they are reduced to butting 

 each other with the crowns of their heads. This causes an 

 additional flow of fluids to that region, with a deposit of 

 horny matter : hence the development of horns and antlers. 



Giraffes live in barren countries, where the only available 

 food is in the leaves of trees. By constantly stretching 

 their necks to reach these leaves, they acquire the strange 

 conformation characteristic of them. Kangaroos Hkewise 

 acquire a powerful pair of hind-legs and tail. 



The sloth is drawn upon likewise for an explanation of 

 its slovenly habits. Originally a dweller on the ground, 

 and then capable ^of moving with the ordinary rapidity, 

 it took to climbing trees, where food was found in such 

 abundance as to require very little movement. More move- 

 ment than necessary would be injurious, partly owing to 

 the heat of the climate in which it lives, and partly because 

 by sitting still and doing nothing it would run fewer risks 

 of falling off and hurting itself. By constant disuse of its 

 locomotive structures, continues Lamarck, it gradually lost 

 the power of moving on the ground, where it is said to be 

 able to take not more than flfty steps in a day. 



Lamarck even adduces an instance from man himself. 

 One, Tenon, had recently stated that the intestine of habitual 

 drunkards is greatly shortened. Now habitual drunkards 

 (he continues) consume a smaller quantity of solid food than 

 ordinary people : there would be less work for their intestine 

 to do, and a corresponding diminution in its length. Such 

 are the facts on which Lamarck bases his theory of the 

 inheritance of functionally-produced modifications. 



Now it is quite clear that all these facts can be explained 

 as easily or more easily on the theory of natural selection, 

 than on that of the inheritance of acquired characters. If 

 it is true that rapid moving among trees is dangerous to 

 sloths, then natural selection would soon ordain that all 



