REGENERATION 21 



cells on the posterior wall and the margin of the iris. And it is 

 certainly not the absence of the lens which gives rise to its renewal, 

 as would necessarily be the case if it were due to the dominance of an 

 adaptive force. If the lens, instead of being excised, be simply 

 pressed back into the vitreous humour occupjnng the cavity of the 

 eye, a new lens is developed all the same from the irritated margin of 

 the pupil. And if by chance this margin has been irritated in two 

 places while extraction of the lens was being performed, then two 

 small lenses will develop (Fig. 99, B). Indeed, several may begin to 

 develop at the posterior wall of the iris, although they do not attain 

 to full development ; mechanical irritation of any part of this cell- 

 layer is responded to by the formation of lenses. This surely disposes 

 of the ' mystical nimbus ' which would dazzle us with a new force of 



Fig. 99. Regeneration of the lens in the Newt's eye. A, section through 

 the iris (/) ; from its margin and posterior (retinal) surface the primordium 

 of a new lens (i) has developed after the artificial removal of the old one. 

 B, section through the eye after duplicated regeneration of the lens (i) from 

 two areas of the iris. Gl, vitreous humour. J, iris. C, cornea. B, retina. 

 After Fischel. 



life, always creating what is appropriate. We have before us an 

 adaptation to the liability of newts' eyes to injury, which, like all 

 adaptations, is only relatively perfect, since under the usual conditions 

 of eye injury it gives rise to a useable lens, but under unusual conditions 

 to unsuitable structures. It is exactly the same as in the case of 

 animal instincts, which are all ' calculated ' for the ordinary conditions 

 of life, but, under unusual conditions, may operate in a manner quite 

 unsuited to the necessary end. The ant-lion has the instinct to 

 bore backwards into the sand, and he makes the same backward- 

 pressing movements when placed on a glass plate into which he cannot 

 force the tip of the abdomen. The same is true of the- mole-cricket, 

 which makes its usual digging movements with the forelegs even on 

 a plate of glass. The wall-bee roofs over her cell when she has laid 



Digitized by Microsoft® 



