3 04 GENERAL BIOLOGY 



more usual course and the more primitive. All the devia- 

 tions from this course that we have been studying, have 

 become thoroughly normalized m the races that exhibit 

 them; the methods of development are stereotyped alike 

 for all. Whatever the course of life, each individual of a 

 species follows it with the most minute exactness. And 

 yet, when something happens to block the usual course, 

 another may be followed, as regeneration and grafting 

 experiments most plainly show, to reach the same end. 

 There are reserves of power for development that the ordinary 

 circumstances of life do not draw upon ; accidents and losses 

 reveal their existence. If a member be maimed and a por- 

 tion of its tissues be injured beyond repair, the injured part 

 must be removed and new tissue fashioned in its stead. 

 Phagocytes enter a wound to clear away old materials, and 

 the blood brings new materials to be gradually fashioned 

 into the form of the old. This is artificial regeneration; 

 but nature makes use of these same pathologic methods in 

 the removal of old tissues and the building of new in meta- 

 morphosis. 



That the functional activity of certain parts of organisms 

 may be increased by selection is shown by the increased milk 

 production of the best dairy breeds of cattle, and by the 

 increased egg production of fowls, etc. Selection has made 

 the dairy cow an improved machine for turning hay 

 and ensilage into milk. But nature presents examples of 

 the exaggerated activity of special functions yet more 

 striking. One such has been fittingly described by Lloyd 

 Morgan in the following words : 



"There is perhaps, no more wonderful instance of rapid 

 and vigorous growth than the formation of antlers of deer. 

 These splendid weapons and adornments are shed and 

 renewed every year. In the spring when they are growing, 

 they are covered by a dark skin, provided with short, fine, 



