104 THE AMERICAN NATURALIST [Vol. XL VII 



Finally correlation has greatly enhanced the value of 

 the old idea of checks in development and shows most 

 clearly that no organ of the body ever reaches its full 

 potentialities. What comes out of an egg is but one of 

 the infinite potentialities contained in it. Velocity of de- 

 velopment, like every other chemical reaction, is equal to 

 the affinity divided by the resistance. If resistances are 

 increased, or if vitality, in other words chemical affinity, 

 be reduced, the development must stop sooner than 

 normal; and we have the phenomena of reversion. If, 

 on the other hand, the reverse takes place, if vitality is 

 increased or resistance reduced, we have variation in the 

 direction of evolution. The development of nonviable 

 monsters is at one extreme of this process. Ontogeny is 

 like a runner, taking the first hurdles easily, but always 

 with increasing difficulty, sometimes trippling at one, 

 sometimes at another, but never reaching the end of his 

 race. 



In conclusion then : to the physiologist it appears that 

 the best explanation of adaptation is that given by Dar- 

 win of natural selection of small variations; that the 

 essential unity of the progress in ^volution toward con- 

 sciousness and intelligence has been due to the natural 

 selection of the fundamental property of irritability, for 

 it is in virtue of this property that adaptability of organ- 

 isms has been increased. The recognition of this fact re- 

 moves one of the difficulties in the way of Darwin's the- 

 ory. And, second, physiology by the establishment of the 

 physiological correlation of all parts of the body, hard 

 and soft, interposes a final objection, in my opinion, to 

 the whole theory of unit characters, of independent vari- 

 ability of characters, and to the theory of evolution in 

 any other way than by a slow and gradual process, which 

 shall give time to the readjustments of every part of the 

 body necessitated by a change, however slight, in any 

 part of it. 



