no The Insides and the Wor\ings of Living Things 



dant concrete examples. In recent years there have been 

 added to the evidences of the observers increasing evidence 

 from the experimental laboratory. It can be shown, for ex- 

 ample, that the formation of the vertebrate eye proceeds 

 from two distinct structural origins in the embryo. One of 

 these produces the sensitive region or retina as a protrusion 

 from the brain, forming the so-called optic vesicle. The other 

 gives rise to the refracting apparatus or lens as an ingrowth 

 from the skin. By removing the optic vesicle early in an 

 animal's development, it is possible to prevent the formation 

 of the uninjured skin area into a lens. The removal of the 

 skin, however, does not interfere with the development of 

 the optic vesicle, nor does the separation of the bud from the 

 brain. If now such an optic vesicle be removed from a 

 young salamander tadpole and engrafted under the skin in 

 another part of the body, the skin overlying the vesicle will 

 develop into a perfect lens. Or again, if the skin above the 

 vesicle is removed and a bit of skin from another part of 

 the body is grafted above the optic vesicle, the foreign skin 

 will develop into a lens. Experiments pointing to the in- 

 fluence of physico-chemical factors upon development have 

 revealed the plasticity of organisms and especially the in- 

 herent capacity for adaptive modification. 



Analogies and Homologies 



From the point of view of evolution, the interesting 

 principle of analogy would not be significant taken by itself, 

 since no matter how a given species came into existence, the 

 individual plant or the individual animal can maintain itself 

 only if it has structures through which it can carry on the 

 functions essential to keeping alive. Taken in conjunction 

 with the fact of homology, however, it is of first importance. 

 Let fishes and lobsters breathe through gills; let the trees of 

 the wood breathe through their leaves: but what should the 

 air-breathing birds and mammals have to do with gills? Or 

 why should herbs use their breathing leaves for catching 



