THE ORIGIN OF VARIATIONS 229 



conceivable 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- 

 dividuals' 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 hun- 

 dred or more generations of successive blacksmiths have been 

 studied. The imagination fails in trying to account for the 

 origin of the lobster's chela te appendages through use and 

 inheritance, but readily conceives how such an organ might 

 have arisen by mutation and 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 functional, 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 in the past have been 

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

 of little use apparently even to Hipparion, have entirely dis- 

 appeared in the modern horse. The digestive tract of intestinal 

 parasites shows various degrees of degeneration through disuse. 

 In some of the thread worms (nematodes) it consists in part of a 



