406 General Biology 



in that certain line of development even though it kill the 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 sub- 

 stances it did when young, continue to grow with the same undiminished 

 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. 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 normal 

 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 

 working 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 is no single 

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

 offspring from some other organism. 



