EVOLUTION 405 



individual. An example which comes to mind is the teeth of rodents. 

 These, when the animal becomes older and is unable to chew the hard 

 substances it did when young, continue to grow with the same undimin- 

 ished vigor that they did when they were constantly being worn down 

 by contact with hard substances, so that they may, as in the beaver, force 

 the mouth open and starve the animal to death. 



But Orthogenesis must be explained, and various reasons have been 

 suggested to account for it, the reasons varying according to the "phi- 

 losophy of life" of the one who is doing the explaining. 



Those who hold that all things are to be explained in terms of physics 

 and chemistry, attempt to explain orthogenesis in physico-chemical 

 terms, while those who hold that there is an inner driving force in all 

 living matter which cannot be explained by physics and chemistry, insist 

 that it is this inner driving force, or "vitalistic principle," which alone can 

 account for it. 



Both sides, however, agree that the cause for this development is 

 not in the organism's environment, but must be sought for in the organ- 

 ism itself. As Professor Borradaile puts it, "The part of the environment 

 is to decide which of the experiments of the organism are failures." And 

 there is sufficient evidence to accept his statement. For instance, the 

 fertilized egg-cells of nearly all the higher organisms are quite alike, yet 

 they develop quite differently. And they retain this difference in devel- 

 opment, even if the egg is transplanted into the body of a different kind 

 of animal and is there allowed to develop to maturity. There must be 

 a difference in the environment of the organs within the bodies of differ- 

 ent animals, yet the egg grows on as it would have done under its nor- 

 mal environment. 



When such definite direction takes place it is called purposiveness. 

 The objection raised against the physico-mechanists (those who be- 

 lieve that all things can be explained in terms of physics and chemistry) 

 by those who do not hold to their point of view (vitalists) is that the 

 body cannot be accepted as a machine in any true sense of the word. A 

 machine produces a very definite and single type of work, while the 

 living organism has all its work directed toward its own welfare, and 

 unlike any machine known, can, when it is injured, direct its entire work- 

 ing system toward repairing itself in addition to continuing its regular 

 work. It not only heals the wound inflicted, but actually grows new 

 material, as we have seen by the regeneration experiments in Planaria 

 and Arthropoda. 



Then, too, there is a decided chasm dividing living and non-living 

 matter ; so much so, that it is a common dictum of biology that abso- 

 lutely no life can come from non-living matter, there being no single 

 case on record of any organism coming into existence except as the off- 

 spring from some other organism. 



However, as we know living things did not exist always, there must 



