NATURAL SELECTION SINCE DARWIN 89 



son for supposing that an accidental &:<! insignificant 

 variation is always accompanied by other variations 

 which lend it usefulness ; and therefore an isolated va- 

 riation could become not only useless but even harm- 

 ful. 



If to the mighty antlers of a stag there did not 

 correspond a special development of the skull and of 

 the muscles of the head and neck, those antlers would 

 only prove an impediment to the animal. Spencer 

 dwells upon the question at length, giving many illus- 

 trations, and concludes that whatever the hypo- 

 thetical processus of parallel modifications may be 

 (simultaneous increase or decrease preserving the 

 original proportions between parts; independent in- 

 crease or decrease modifying those proportions; va- 

 riations of such nature that in the end the various 

 structures are adapted to a new purpose) mere nat- 

 ural selection cannot be considered as a satisfactory <r 

 solution of the problem. 



Unless we believe in a pre-established order of 

 things, we are compelled, Spencer believes, to adopt 

 the one plausible explanation: structural modifica- 

 tions caused by functional modifications in each indi- 

 vidual and transmitted to a certain extent to its de- 

 scendants. Then all co-adaptations, from the simplest 

 to the most complex, become intelligible. In certain 

 cases, inheritance of acquired characters suffices to 

 explain all facts; in other cases it explains them only 



