452 THE AMERICAN NATURALIST [Vol.L 



is, however, essentially Nageli's theory of evolution from 

 within by virtue of a perfecting or progressive tendency, 

 in support of which Xageli himself draws the parallel 

 between embryology and evolution. It has certain points 

 of resemblance to Elmer's orthogenesis, but differs from 

 it tremendously in that Eimer thought evolution was 

 directed by the external world and that there were summed 

 in the germ plasm the impressions of that world received 

 in successive generations. Huxley was inclined to accept 

 the theory of internal factors in evolution. He says : 



I apprehend that the foundation of the theory of natural selection is 

 the fact that living bodies tend incessantly to vary. This variation is 

 neither indefinite, nor fortuitous, nor does it take place in all directions, 

 in the strict sense of these words. ... A whale does not tend to vary 

 in the direction of producing feathers, nor a bird in the direction of 

 developing whalebone. 



Mivart and many others have long contended that evolu- 

 tion is due to internal factors. Their views are in accord 

 with Aristotle's. 



Finally, I may close this section by quoting a simile 

 from Bergson, as translated by Mitchell. 



The evolution movement would be a simple one, and we should soon 

 have been able to determine its direction, if life had described a single 

 course, like that of a solid ball shot from a cannon. But it proceeds 

 rather like a shell, which suddenly bursts into fragments, which frag- 



tined to burst again, and so on for a time incommensurably long. We 

 perceive only what is nearest to us, namely, the scattered movements of 



When a shell bursts the particular way it breaks is explained both by 

 the explosive force of the powder it contains and by the presistance of 

 the metal. So of the way life breaks into individuals and species. It 

 depends, we think, on two series of causes: the resistance life meets 

 from inert matter, and the explosive force— due to an unstable balance 

 of tendencies— which life bears within itself. 



II. SrjPPOKT FOE THE THEORY FROM COLLATERAL FlELDS 



The vi-ew that the course of evolution (like the develop- 

 ment of the individual) is chiefly determined by internal 



