256 THE ORIGIN OF SPECIES 



exterior surface will then digest and the stomach respire, 

 lu such cases natural selection might specialize, if any 

 advantage were thus gained, the whole or part of an 

 organ which had previously performed two functions for 

 one function alone, and thus by insensible steps greatly 

 change its nature. Many plants are known which regu- 

 larly produce at the same time differently constructed 

 flowers; and if such plants were to produce one kind 

 alone, a great change would be effected with compara- 

 tive suddenness in the character of the species. It is, 

 however, probable that the two sorts of flowers borne 

 by the same plant were originally differentiated by finely 

 graduated steps, which may still be followed in some 

 few cases. 



Again, two distinct organs, or the same organ under 

 two very different forms, may simultaneously perform 

 in the same individual the same function, and this is 

 an extremely important means of transition: to give one 

 instance — there are fish with gills or branchiae that 

 breathe the air dissolved in the water at the same 

 time that they breathe free air in their swimbladders, 

 this latter organ being divided by highly vascular parti- 

 tions and having a ductus pneumaticus for the supply of 

 air. To give another instance from the vegetable king- 

 dom: plants climb by three distinct means, by spirally 

 twining, by clasping a support with their sensitive ten- 

 drils, and by the emission of aerial rootlets; these three 

 means are usually found in distinct groups, but some few 

 species exhibit two of the means, or even all three, com- 

 bined in the same individual. In all such cases one of 

 the two organs might readily be modified and perfected 

 so as to perform all the work, being aided during the 



