234 THE PERPETUATION OF ADAPTATIONS 



the mutant type, until possibly the ancestral or original type is 

 crowded out. Thus by natural selection, a new species and one 

 better adapted to the environment would result. Or it is con- 

 ceivable that dominance may shift about with environmental 

 changes, thus leading to change of type. Speculations as to 

 the origin of variations based on the theory of mutations are 

 endless, and there is apparently good ground for many of them. 



The Inheritance of Acquired Characteristics. Beginning with 

 Lamarck (1744-1829), many biologists have held that changes 

 brought about in structures of the individual, during that in- 

 dividual's lifetime, are transmitted by inheritance to the off- 

 spring. This conception, extremely difficult to prove experi- 

 mentally, involves the fundamental principle of use and disuse 

 of organs as affecting the descendants and the race. 



Everyone knows that continued use of an organ strengthens 

 it a time-worn illustration is the blacksmith's arm but there 

 is no evidence that this over-developed organ is transmitted to 

 the offspring, no evidence that the blacksmith's children differ 

 from other children in muscular development. This point could 

 not be satisfactorily proved, however, until a hundred or more 

 generations of successive blacksmiths have been studied. The 

 imagination fails in trying to account for the origin of the lob- 

 ster's chelate appendages through use and inheritance, but 

 readily conceives how such an organ might have arisen by mu- 

 tation and been transmitted by inheritance, and which, being 

 useful, is preserved in the race by natural selection. On the other 

 hand, the effects of use seem to be shown in the single-toed horse 

 of today, which has descended from ancestral forms having four 

 toes and a rudimentary fifth (Eohippus) on the front legs, and 

 from forms having three toes, one of which is large and func- 

 tional, the other two reduced (Hipparion, Merychippus) . This 

 appears to be a case where continued use has resulted in the 

 modern structure. 



The effect of disuse may be readily imagined. Vestigial or- 

 gans are evidence of structures which hi the past have been use- 

 ful in one way or other. The lateral toes of the fossil horse, of 

 little use apparently even to Hipparion, have entirely disap- 



