PHYSIOLOGICAL (OR CONSTITUTIONAL) UNITS. 615 



Such evidence as we have verifies this interpretation. There is 

 first the universal fact that development of the germ begins 

 when it is exposed to an incident force heat the undulations of 

 which, increasing the oscillations of the mixed units, give them 

 greater freedom to arrange themselves in conformity with their 

 type. We see this alike when spring warmth makes a seed 

 germinate and when the warmth of a sitting hen sets up organiza 

 tion in her eggs. Heat frees the molecules of inorganic matter 

 from local restraints and, as we see in molten metal, lets them 

 yield to other forces ; and similarly in this organic matter, the 

 units are made free to follow their proclivities. Then, secondly, 

 there comes the evidence from comparisons between the effects of 

 mixing constitutional units differing in various degrees. Let the 

 cluster of mixed units be derived from animals that are ordinally 

 distinct. Nothing happens. The units each contributes tend to 

 arrange themselves after the parental type. Hence a conflict 

 between the tendencies towards two markedly unlike structures, 

 and no structure arises. Suppose the mixed units come from two 

 kindred species say horse and ass. The structures which they 

 respectively tend to form, being in their main characters alike, 

 there is such cooperation as produces a working organism but an 

 organism in certain respects imperfect a mule. Suppose, again, 

 the units come from two varieties of the same species. A perfect 

 organism results, and, as shown by Mr. Darwin when detailing 

 the effects of crossing, an unusually vigorous organism. The 

 units being more unlike than those belonging to the same variety, 

 the instability of the germ-plasm is unusually great, and the 

 transformations which constitute development and action become 

 unusually active. When, as in ordinary cases, the units are sup 

 plied by members of the same variety who have not been made 

 very much alike by their antecedents, there follows the usual 

 amount of organic vigour. Coming now to the results of breed 

 ing in-and-in breeding between individuals whose constitutions 

 (i.e. constitutional units) have for generations been growing more 

 alike in the absence of crossing with other stirps we see that 

 diminution of organic vigour is displayed : there is a decrease in 

 the rate of physiological change. Finally, on coming to a closer 

 relationship, as in marriages between cousins, in whom the 

 constitutional units are more than commonly alike, we see there 

 frequently follows either barrenness or the production of feeble 

 offspring. 



All these facts, then, are congruous with the hypothesis that 

 the use of fertilization is the mixing of unlike units, and conse 

 quent production of plasticity. Leaving out cases in which the 

 unlikenesses are so great as wholly to prevent cooperation among 

 the units, the degree of vigour, that is, the activity of physiologi- 



