16 MANUAL OF ELEMENTARY ZOOLOGY 



to be so adapted to its circumstances that the efficiency of its purposive 

 activity is increased. 



These features in the activity of living beings place them 



in a relation to their surroundings which is 

 Ma'hine!* entirely different from that of lifeless things. 



The forces of nature acting from without upon 

 a lifeless object find in it a toy or a passive victim. In the 

 living being they set in action a machine of great efficiency 

 which reacts upon them for its own benefit. In so doing, 

 it is engaged in the " struggle for existence." The result of 

 this is, in the animal, ability to exist amid surroundings 

 which would destroy it but for its life. In the world as a 

 whole the result is a complication of the action of its forces 

 which is a factor of enormous importance in the system of 

 nature. By the living machine these forces bring about 

 results which they could not otherwise accomplish. The 

 history of this reaction is written large in the very substance 

 of the earth. Enormous beds of chalk and limestone com- 

 posed of the skeletons of minute marine animals, countless 

 coral reefs and islands, vast areas covered with vegetable 

 mould by theaction of plants and earthworms, andgreat tracts 

 of country whose face has been changed by human activity 

 bear witness to its existence ; and since the coming of Man 

 it has progressed more and more rapidly till it promises 

 to dominate every other terrestrial agent of change in 

 nature. 



We are now in a position to sum up the characteristic 



features of the complex process which is known 

 summary of as life. In doing so we shall arrange them in a 

 iatios of Life. r " somewhat different order from that in which they 



have come under our notice, stating successively 

 those that relate to the starting and stopping of the life pro- 

 cesses, to the nature of these processes, and to the end to 

 which they tend. We have found in life the following 

 features : 1 — 



1 The process of respiration is not included in the following list 

 because, though it is often rightly cited as a characteristic feature of 

 the life of animals, it is not a simple or distinct process. It consists in 

 the excretion of carbon dioxide and the taking up of oxygen by a process 

 which is not in its essence different from the incorporation of other 

 materials. Conjugation is rejected because it is not a universal 

 property of living matter. 



