xlii INTRODUCTION 



sloths should become inactive. Rash sloths, which moved 

 about rapidly and hence fell to the ground, would be injured 

 and more or less incapacitated for reproduction. They 

 therefore leave no progeny of rash sloths, similar to them- 

 selves. Cautious sloths, on the other hand, which move 

 as little as possible, run fewer risks of falling off and getting 

 hurt ; they are therefore likely to leave progeny, and this 

 progeny will consist of cautious sloths like their parents. 

 Thus there is a perpetual tendency for the species to be 

 recruited from the cautious and prudent sloths, and for the 

 extinction of the rash sloths. In course of time, none but 

 cautious sloths will remain. 



The case is similar with the giraffe's neck. In time of 

 stress, giraffes with unusually long necks will have access to 

 a better supply of food than giraffes with short necks. The 

 latter will die out, while the former will survive. On this 

 assumption, there is no necessity for any inheritance of 

 acquired characters. The unusual length of neck is a 

 spontaneous congenital variation, arising by pure chance ; 

 it is preserved and inherited. 



From the à priori view, there does not seem a great deal 

 to choose between the theories ; and it was the à priori view 

 that was adopted by Lamarck. Many of the known facts of 

 evolution might be accounted for either by use-inheritance 

 or by natural selection. If it is true that acquired characters 

 are inherited, then the giraffe might well have developed his 

 neck through that agency. The hypothesis fits the facts. 

 But so also does the hypothesis of special creation ; for if 

 God manufactured the giraffe, neck and all, just as we find 

 him, we immediately reach the goal of our researches on the 

 matter. Similarly, again, "natural selection is equally satis- 

 factory as an à priori hypothesis. If congenital variations 

 are inherited, and if favourable variations have a real sur- 

 vival-value for the individual, then natural selection might 

 well have been the true method. 



The fact is, of course, that the above mode of reasoning 

 is a grave abuse of the deductive method. Few, indeed, are 



