INFLUENCE OF ENVIRONMENT 119 



and such that these organs would burst rather than yield at once to 

 any demand for the ordinary extension. 



Compare two men of equal ages, one of whom has contracted the 

 habit of eating very little, since his habitual studies and mental work 

 have made digestion difficult, while the other habitually takes much 

 exercise, is often out-of-doors, and eats well ; the stomach of the first 

 will have very little capacity left and will be filled up by a very small 

 quantity of food, while that of the second will have preserved and 

 even increased its capacity. 



Here then is an organ which undergoes profound modification in 

 size and capacity, purely on account of a change of habits during the 

 life of the individual. 



The frequent use of any organ, when confirmed by habit, increases the 

 functions of that organ, leads to its development and endows it with a 

 size and power that it does not possess in animals which exercise it less. 



We have seen that the disuse of any organ modifies, reduces and 

 finally extinguishes it. I shall now prove that the constant use of 

 any organ, accompanied by efforts to get the most out of it, strengthens 

 and enlarges that organ, or creates new ones to carry on functions 

 that have become necessary. 



The bird which is drawn to the water by its need of finding there 

 the prey on which it lives, separates the digits of its feet in trying to 

 strike the water and move about on the surface. The skin which unites 

 these digits at their base acquires the habit of being stretched by these 

 continually repeated separations of the digits ; thus in course of time 

 there are formed large webs which unite the digits of ducks, geese, 

 etc., as we actually find them. In the same way efforts to swim, 

 that is to push against the water so as to move about in it, have 

 stretched the membranes between the digits of frogs, sea-tortoises, 

 the otter, beaver, etc. 



On the other hand, a bird which is accustomed to perch on trees 

 and which springs from individuals all of whom had acquired this habit, 

 necessarily has longer digits on its feet and differently shaped from those 

 of the aquatic animals that I have just named. Its claws in time be- 

 come lengthened, sharpened and curved into hooks, to clasp the 

 branches on which the animal so often rests. 



We find in the same way that the bird of the water-side which does 

 not like swimming and yet is in need of going to the water's edge 

 to secure its prey, is continually liable to sink in the mud. Now this 

 bird tries to act in such a way that its body should not be immersed 

 in the liquid, and hence makes its best efforts to stretch and lengthen 

 its legs. The long-established habit acquired by this bird and all 



