CHAPTER X. 



SUMMARY OF PHYSIOLOGICAL DEVELOPMENT. 



310. INTERCOURSE between each part and the particular 

 conditions to which it is exposed, either habitually in the 

 individual or occasionally in the race, thus appears to be the 

 origin of physiological development ; as we found it to be the 

 origin of morphological development. The unlikenesses of 

 form that arise among members of an aggregate that were 

 originally alike, we traced to unlikenesses in the incident 

 forces. And in the foregoing chapters we have traced to 

 unlikenesses in the incident forces, these unlikenesses of 

 minute structure and chemical composition that simulta- 

 neously arise among the parts. 



In summing up the special truths illustrative of this 

 general truth, it will be proper here to contemplate more 

 especially their dependence on first principles. Dealing with 

 biological phenomena as phenomena of evolution, we have to 

 interpret not only the increasing morphological heterogeneity 

 of organisms, but also their increasing physiological hetero- 

 geneity, in terms of the re-distribution of matter and motion. 

 While we make our rapid re-survey of the facts, let us then 

 more particularly observe how they are subordinate to the 

 universal course of this re-distribution. 



311. The instability of the homogeneous, or, strictly 

 speaking, the inevitable lapse of the more homogeneous into 

 the less homogeneous, which we before saw endlessly exem- 



