38 ORGANIC EVOLUTION 



on the earth. Also the principle of correlation between 

 organs is important here. Organs not useful in them- 

 selves may be correlated with other organs of great value 

 and be developed and perfected along with these until 

 they reach a degree of development that renders them 

 themselves useful. 



There is another important principle that helps us under- 

 stand the beginnings in the evolution of useful structures and 



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habits. If some organ is to be developed to meet some new 

 need, it is rarely, if ever, formed from a previously un differ- 

 entiated part of the organism, but is rather formed by modi- 

 fication of some organ already present, the change in this 

 organ fitting it for a different use, fitting it to meet the new 

 need. Similarly if a new habit needs to be acquired, it is 

 likely to arise as a modification of some previous habit. 

 The different stages in the evolution of an organ may each 

 be useful for a different purpose. In fact it is probable that 

 the orsfan in its several conditions will serve somewhat 



O 



different purposes. One can hardly mention an organ in 

 the human body, for example, which has not in this way 

 been changed in its function. The heart was once a simple 

 blood vessel, serving for the carriage of blood, not for its 

 propulsion ; the lungs were, in the fishes, the swim-bladder, 

 which became changed into an air-breathing organ as the 

 terrestrial habit was acquired ; the limbs in the early aquatic 

 vertebrates probably were used as guides and balancers in 

 swimming and as swimming paddles ; later, as the terrestrial 

 habit was acquired, they assumed a form adapted for locomo- 

 tion on land. Chancre of function and change of structure 



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go hand in hand, so that the different stages in the evolution 

 of an organ do not all serve the same purpose. Hair was 



