10 NATURAL HISTORY. 



tions of animal life. In the other animals a group of cells will have 

 certain duties to perform, and will be unconcerned in all others. In the 

 lower forms one of these cell-groups may perform several functions, but 

 as we ascend in the animal scale we mid a constant specialization and 

 differentiation of cells and cell-groups ; and just in this respect do we 

 speak of one animal being higher or lower than another. Looked at 

 from the standpoint of adaptation of means to ends, one is as perfect 

 as the other ; but in one the subdivision of labor is carried to a greater 

 extent than in the other, and this is largely the criterion of rank. 



