THE MODIFICATION OF ORGANS AND FUNCTIONS 29 



richly supplied with blood-vessels. This respiratory mem- 

 brane is not, in the crayfish, situated inside the body, but 

 on the outer surface, although protected by being in a sort 

 of pocket with a covering flap, and it comes into immediate 

 contact with the air held in the water which freely bathes 

 the gills. By osmosis the oxygen of this air passes in 

 through the gill-membranes, while the carbonic acid gas 

 brought by the blood passes out through them. Exactly 

 the same exchange of gases is accomplished as in the 

 toad. But because of the great difference in the conditions 

 of life of the toad and crayfish, one living in \vater, the 

 other living out of water, the character of the performance 

 of the function of respiration, and correspondingly the 

 structural condition of the organs performing this function, 

 are strikingly different. 



Modification of functions and structure to fit the 

 animal to special conditions of its life. As has been 

 done with the organs of digestion, circulation, and 

 respiration, so we might compare the other organs of the 

 crayfish and the toad. There would be found not only 

 many very marked differences between organs which have 

 the same general function in the two animals, but we 

 should find also numerous organs in the toad which are 

 not present at all in the crayfish, and conversely; and this 

 means, of course, that the toad can do numerous things, 

 perform numerous functions, which the crayfish cannot, 

 and, conversely, that the crayfish does some things which 

 the toad cannot. But both of these animals agree in 

 possessing in common the capability of performing those 

 processes such as taking food, breathing, reproducing, 

 etc., to which attention has been called as being indis- 

 pensable to all animal life. These processes, however, 

 are performed by the two animals in different ways and 

 the organs for the performance of these processes, although 

 at very bottom essentially alike, are in outer and super- 



