?88 IHE INDUCTIONS OF BIOLOGY. 



structure, and which are at the same time slowly modifiable 

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

 tiplication 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 simple organisms, lost parts can be re- 

 placed, and that, in still simpler organisms, a fragment can 

 develop into a whole. 



While an aggregate of physiological 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 between 

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

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

 siological 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-difier- 

 entiated organisms, so long as assimilation continues to be 

 greatly in excess of expenditure. 



But let growth be checked and development approach its 

 completion — let the units of the aggregate be severally exposed 

 to an almost constant distribution of forces ; and they must 

 begin to equilibrate themselves. Arranged as they will 

 gradually be, into comparatively stable attitudes in relation 

 to each other, their mobility will diminish ; and groups of 

 (hem, partially or wholly detached, will no longer readily re- 

 arrange themselves into the specific form. Agamogenesis wiU 

 be no longer possible ; or, if possible, will be no longer easy. 



When we remember that the force which keeps the Earth 

 in its orbit, is the gravitation of each particle in the Earth 

 towards every one of the group of particles existing 91,000,000 

 r>f miles off; we cannot reasonably doubt, that each unit in 



