EVOLUTION— PYCRAFT 221 



that of the kitchen oven. Wliy should the products of digestion 

 have brought about this singular "ear-marking" of these two breast 

 muscles ? 



Let me give one more illustration, to which, indeed I draw your 

 very particular attention. A solution of racemic acid subjected to 

 the plane of a beam of polarized light does not give the effect known 

 as "polarization." But if allowed to crystallize, rhombic, hemihedral 

 crystals are formed. These are asymmetrical, so much so that the 

 crystals of one group are like mirror-images of the other group. 

 Now if these crystals are dissolved separately, and the two solutions 

 examined under this test, they exhibit what is known as rotary polar- 

 ization, in the one case the plane is twisted to the right, in the other to 

 the left. 



But if instead of crystallizing the original solution the spores of 

 the green mold Penicillium be sown in it, and that solution be then fil- 

 tered, it will exhibit only left-handed rotary polarization. The fungus 

 has selected the right-handed moiety for the purposes of its growth, 

 leaving as a residuum only that constituent of the solution which 

 forms left-handed crystals. Here, then, is a striking example of the 

 subtle processes of distillation which living substances are capable of, 

 in building up new tissues, either to repair waste, or to promote 

 growth. What part have "natural selection", or the "enviromnent", 

 played in any of these instances? 



Now let me pass to survey the evidence which, so it seems to me, 

 proves the immense importance of "use" as a factor in molding ani- 

 mal bodies, for of necessity I must exclude plants, though the same 

 factors obtain here. 



The persistent habit of burrowing, in some ancestral shrew gave rise 

 at last to the mole. The stimuli of persistent use, in one direction, 

 caused those parts of the body more especially subjected to such stim- 

 uli to change their shape, but exactly in accordance with the peculi- 

 arities of the qualities of the tissue peculiar to the body of the mole. 

 The marsupial-mole, an animal not even remotely related to our mole, 

 has similarly transformed the fore limbs into digging organs, but 

 they differ conspicuously from those of the common mole. 



With these preliminary examples to show what I am driving at, 

 and to enable you to follow me more easily, let me proceed to take a 

 series of examples of different types of animals "in the making." I 

 will begin with the arboreal types (pi. 1). 



Birds and beasts of many different kinds make their home among 

 trees, as do others on the ground. But some have become intensively 

 arboreal, and this habit is always accompanied by more or less strik- 

 ing adjustments of structure brought about in response to the per- 

 sistent and restricted use of the parts affected. But these adjustments 



