TRACHEATES 2O9 



When from any cause the antlers of the stag became 

 larger, they exercised a pressure on the skull, which led 

 to its thickening, and the other parts of the body would 

 be equally strengthened and modified by the pull of their 

 burden. These variations were transmitted to offspring, 

 and if a still stronger pair of antlers arose in the next 

 generation, and was selected, it found better support, and 

 would in turn improve this by its pressure, and so 

 the advance would gradually continue. 



This kind of explanation seems simple enough, but 

 there are certain co - adaptations that it cannot cover. 

 This is the case with the insects once more. We have 

 already seen that the cutaneous skeleton of these 

 animals is dead, and can only be used up, not 

 strengthened, by exercise. Now, the peculiar fiddling 

 apparatus of the field-cricket only makes its appearance 

 at the last cast of the skin. It consists of two very 

 different parts — a bow, which is represented by a 

 specially modified nervure of the wing, and the side 

 across which it is drawn. The latter is a part of the 

 inner surface of the hind leg, which is equipped with a 

 number of little teeth ; it is these that give out the 

 chirping sound when they are rubbed. Here we have 

 a co-adaptation that the Lamarckian principle cannot 

 explain. Two organs that lie in different parts of the 

 body are modified in the same direction, so that one 

 can only co-operate with the other ; but they can neither 

 have arisen nor been improved by use. They only 

 make their appearance at the last cast of the skin, and 

 when they rub together they only wear themselves, 

 since they are dead structures, and the underlying 



