I 3 2 ANIMAL LIFE 



now, let that organ be taken from some other animal, 

 say a sheep, and its extract be given in doses to the 

 dwarfed being, there will often result a gradual change. 

 The needful stimulus is taken up by the blood, carried 

 to the brain and body, and there evokes the hidden 

 and arrested growth of the stunted parts. 



In such a way many organs play their part by 

 exciting the activity of others, perhaps far away 

 from them ; and to this principle we can attribute the 

 strange and orderly persistence of organs whose 

 primary use has disappeared. The gill-slits, for 

 example, which recur with unfailing regularity in the 

 development of all vertebrate animals, are only used 

 in their primitive sense by fish. But in higher animals 

 their walls give rise to thyroid and other glands that 

 serve to excite the growth of the neck and larynx, and 

 without these glands and gill-bars, which in fish have 

 a totally different function, our development would be 

 imperfect and retarded. Or, to take perhaps the most 

 striking case the change from youth to manhood, 

 and the corresponding change in animals a stimulus 

 affects the whole body, alters the voice, broadens the 

 chest, beards the face, and intensifies the conduct. 

 In animals, as we have seen, so great is this change 

 that the colour and eyes, the limbs and skin, undergo a 

 complete metamorphosis : their movements and habitat 

 are exchanged for new ones ; the bird, fish, insect, and 

 even certain worms, become different beings. When 

 the change is less radical, as in the growth of antlers 

 and resonance of voice or ferocity of manner, we 



