76 FOOT-NOTES TO EVOLUTION. 



Its influence is shown in the persistence of type, in the 

 existence of broad homologies among living forms, in 

 the possibility of natural systems of classification in 

 any group, in the retention of vestigial organs, in ttye 

 early development and subsequent obliteration of out- 

 worn structures once useful to the race or type. 



The physical basis of heredity has been in recent 

 years the subject of many elaborate investigations. The 

 complete homology of the germ cell with the one-celled 

 animals, or protozoa, is now generally recognised, and 

 there is large reason to believe that in the bands and 

 loops of the nucleus of the germ cell is found the visible 

 vehicle by which hereditary tendencies are transmitted. 



II. Irritability. All living beings are affected by 

 their environment. Living matter must always respond 

 in some degree to every external stimulus. All living 

 beings are moved by or react from every phase of their 

 surroundings. The nervous system and its associated 

 sense organs are directly related to the conditions of 

 life. They are concessions made to the environment. 

 The power of motion, whatever it may be, requires the 

 guidance obtained from the impressions made by ex- 

 ternal things. In all animals this knowledge, whatever 

 its degree of completeness, tends to work itself out in 

 action. In plants the same thing is in some degree 

 true. The essential difference is that, having no power 

 of locomotion, the plant is without a general sensorium. 

 The parts that move growing rootlets, tips of branches, 

 and the like have sensibility and power of motion in 

 the same series of cells. The animal, a colony of cells 

 which move as a whole, has a specialized nervous sys- 

 tem which guides the whole. 



As a rule, the environment does not act directly on 

 the individual. Its influence is felt chiefly in modifying 

 its action, in increasing, diminishing, or changing its 



