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 

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

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

 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 organ in its several conditions will serve somewhat 

 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, but, later, as the terres- 

 trial habit was acquired, they assumed a form adapted for loco- 

 motion on land. Change of function and change of structure 

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

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



