GENESIS, HEREDITY, AND VARIATION. 351 



far higher degrees. Far more numerous must be the minute 

 changes that can be wrought in them by minute external 

 forces; far more free must they remain for a long time to 

 obey forces tending to re-distribute them; and far greater 

 must be the number of their kinds. 



Setting out with these physiological units, the existence of 

 which various organic phenomena compel us to recognize, and 

 the production of which the general law of Evolution thus 

 leads us to anticipate ; we get an insight into the phenomena 

 of Genesis, Heredity, and Variation. If each organism is built 

 of certain of these highly-plastic units peculiar to its species 

 — units which slowly work towards an equilibrium of their 

 complex proclivities, in producing an aggregate of the specific 

 structure, and which are at the same time slowly modifiable 

 by the re-actions of this aggregate — we see why the multi- 

 plication of organisms proceeds in the several ways, and with 

 the various results, which naturalists have observed. 



Heredity, as shown not only in the repetition of the specific 

 structure but in the repetition of ancestral deviations from it, 

 becomes a matter of course; and it falls into unison with the 

 fact that, in various inferior organisms, lost parts can be 

 replaced, and that, in still lower organisms, a fragment can 

 develop into a whole. 



While an aggregate of ph3^siological units continues to 

 grow by the assimilation of matter which it moulds into 

 other units of like type; and while it continues to undergo 

 changes of structure; no equilibrium can be arrived at be- 

 tween the whole and its parts. Under these conditions, then, 

 an un-differentiated portion of the aggregate — a group of 

 physiological units not bound up into a specialized tissue — 

 will be able to arrange itself into the structure peculiar to the 

 species; and will so arrange itself, if freed from controlling 

 forces and placed in fit conditions of nutrition and temper- 

 ature. Hence the continuance of agamogenesis in little- 

 differentiated organisms, so long as assimilation continues to 

 be greatly in excess of expenditure. 



