614 APPENDIX F. 



give out much motion. What enables it to do this ? The presence 

 in it of abundant molecules containing much molecular motion 

 which can be transformed into molar motion : the transformation 

 being effected by the falling of these molecules into their simpler 

 and relatively-inert components, which are thereupon excreted. 

 Energy-containing matter is used up, and more energy or vigour 

 can be given only by supplying more such matter. How then 

 can the union of two nuclei — those of the sperm-cell and germ- 

 cell — give vigour ? Only an infinitesimal portion of vigour in the 

 sense above explained exists in either, and the union of them 

 leaves it still infinitesimal. And then, even supposing the vigour 

 to be an entity and to be appreciable in quantity, how could it 

 go on producing that immense -combination of physiological 

 actions seen in the unfolding of the germ into an organism ? and 

 how could it go on producing the physiological actions of an adult 

 organism during a whole century ? 



May we not then say that these proposed explanations leave the 

 question where it was — are nominal solutions, not real solutions ? 



5. But the hypothesis of constitutional units furnishes, if not a 

 satisfactory answer yet, something in the nature of an answer — a 

 true cause ; that is to say, a cause actually known to us as 

 operating in other cases. In § 92 it was pointed out that in 

 proportion as units are similar, there may be built up from them 

 an aggregate which is relatively stable, and that along with in- 

 creasing dissimilarity the stability of the aggregate decreases. It 

 was inferred that if a group of constitutional units belonging to 

 one individual which have become moulded into relatively exact 

 congruity with the organism and with one another by long co- 

 operation, are mingled with some belonging to another individual 

 which, differently circumstanced, has become somewhat different 

 in itself and in its units, then the mass formed by the union of 

 the two groups will be relatively unstable — relatively modifiable 

 by incident forces. Whereas in either organism, no longer per- 

 petually changed in the relations of its parts by growth, there is 

 an approach towards equilibrium between the whole and its com- 

 ponents, the components contributed by the two to form a 

 germ, being slightly unlike one another, will not form a group 

 in a state of equilibrium. The group they form will be capable 

 of easy change by incident forces ; and they will so be rendered 

 free to follow their proclivities towards the typical form of the 

 species. Inferring this we must also infer that so long as these 

 two sets of slightly different units are not exposed to any constant 

 forces tending to coerce them into the same form, there will con- 

 tinue to exist in the nuclei of all descendant cells this same rela- 

 tive instability and consequent plasticity. 



