244 DARWINISM AND THE PROBLEMS OF LIFE 



determine the origin of any organ with certainty, as we 

 have no absolutely certain documents. We construct 

 the development of animals and their organs with the 

 highest probability, once we have established the fact 

 that they must have been evolved. The soundness of 

 the theory of selection is by no means shaken by 

 quoting organs the origin of which we are not at 

 present able to explain. 



We will not attempt at the moment to settle com- 

 pletely the difficulty that arises about the selective 

 value of variations. We will merely refer to a 

 subsidiary principle which provides a simple solution 

 of the difficulties that are found in regard to the 

 usefulness of the first variations of many organs. This 

 is the principle of change of function. 



We became acquainted with this principle in describ- 

 ing the conversion of the swimming-bladder of the fish 

 into the lungs of the amphibia. Something similar 

 happened in the development of the land-snails. In 

 the marine snails the gills lay in a cavity that opened 

 externally, and there were several blood-vessels in the 

 wall of it. In the pulmonary snails also the respiratory 

 organ is a cavity which differs from the preceding one 

 chiefly in the absence of gills, and in the fact that the 

 blood-vessels in its lining are so numerous and ramified 

 that the air can give its oxygen, which penetrates through 

 their thin walls, to the blood. These lungs are one of the 

 best conceivable instances of the gradual transformation 

 of one organ into another. They show, in the first 

 place, that here, where natural selection needed an 

 extensive organ from the first, there was one ready to 



