60 THE THEORY OF CHANGE OF FUNCTION 



and it is readily conceivable that both glands 

 and muscular wall might be better developed in 

 one half of the stomach than the other, and indeed 

 this condition actually occurs in the stomach of the 

 rat. A continuance of this modification brings us 

 to the condition met with in the ruminant stomach. 

 Here the first compartment is a mere receptacle 

 for the storing of food, and in this no digestion 

 takes place at all. Again, from the same starting- 

 point, we have another series. Suppose the 

 glands to aggregate at one end of the stomach, and 

 the muscular coat to be thickened at the other ; if we 

 carry this far enough we get to the condition of the 

 stomach of the bird where peptic glands are con- 

 fined to one part, muscles to the other. Here the 

 second portion has completely lost its original 

 primary function, this having become replaced by 

 a secondary function which has now become 

 primary. 



The modifications of the limbs of Arthropods 

 afford numerous and admirable illustrations of the 

 case before us. Here we have organs whose 

 primary function is undoubtedly locomotion, but 

 of which a certain number, greater or less, in 

 different groups, have become modified so as to 

 aid in the mastication of food. Naturally, those 

 nearest the mouth are the ones so modified, and 

 of these, those at the actual sides of the mouth 

 are most likely to undergo the greatest modifica- 

 tions. Such modifications, if extreme, would unfit 

 them for their primary locomotor function, hence 

 we find the first post oral pair of appendages, 



