g6 The Insides and the Workings of Living Things 



mental identity of wings and legs and arms and flippers unless 

 here too there is a far-reaching kinship, or common origin. 

 The assumption that kinship is somehow indicated by- 

 similarity must be applied here among the different classes 

 of backboned animals, among the different classes of ar- 

 thropods, among the different classes of any main branch 

 of the animal world, just as we apply it to different breeds 

 of dogs or pigeons. We cannot escape the conclusion that 

 there is likeness because there is descent from a common 

 ancestry. 



Correlation 



We have seen that there is a unity in all life in the sense 

 that all living things carry on the same fundamental processes. 

 This means that, on the whole, a plant or animal remains alive 

 as long as it maintains a certain balance between internal 

 processes and external conditions. There must be an income 

 of materials. There must be an internal redistribution of 

 materials. There must be a transformation of rriaterials and 

 a rejection or excretion of certain portions. There must be 

 escape from enemies as well as from unfavorable conditions 

 of moisture, temperature, light or darkness, and so on. Liv- 

 ing things are adapted to the conditions in which they live 

 — they must be, or they could not continue to live. The 

 great variety of life corresponds to the great variety of con- 

 ditions — and the living beings themselves make up an im- 

 portant part of the effective environment for one another. 

 In all the variety there remains the common factor called 

 " protoplasm," a complex mixture of numerous substances — 

 and the common processes. All life is one in essential per- 

 formance and needs, regardless of how it originated. 



The necessity of maintaining certain relationships to 

 the outer world — the physical conditions, the food supply, 

 possible enemies — brings about a unity in the organism 

 that Cuvier emphasized in his doctrine of correlation. A 

 mode of life involves all the factors of the environment and 



