352 THE INDUCTIONS OF BIOLOGY. 



But let growth be checked and development approach its 

 completion let the units of the aggregate be severally ex 

 posed 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 one another, their mobility will diminish; and groups of 

 them, partially or wholly detached, will no longer readily re 

 arrange themselves into the specific form. Agamogenesis will 

 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 92,000,000 

 of miles off; we cannot reasonably doubt that each unit in 

 an organism acts on all the other units, and is reacted on by 

 them: not by gravitation only but chiefly by other energies. 

 When, too, we learn that glass has its molecular constitution 

 changed by light, and that substances so rigid and stable as 

 metals have their atoms re-arranged by forces radiated in 

 the dark from adjacent objects ; * we are obliged to conclude 

 that the excessively-unstable units of which organisms are 

 built, must be sensitive in a transcendant degree to all the 

 forces pervading the organisms composed of them must be 

 tending ever to re-adjust, not only their relative attitudes 

 but their molecular structures, into equilibrium with these 

 forces. Hence, if aggregates of the same species are differ 

 ently conditioned, and re-act differently on their component 

 units, their component units will be rendered somewhat dif 

 ferent; and they will become the more different the more 

 widely the re-actions of the aggregates upon them differ, and 

 the greater the number of generations through which these 

 different re-actions of the aggregates upon them are continued. 



* Fifty years before the discovery of the Rontgen rays .and those habitually 

 emanating from uranium, it had been observed by Moser that under certain 

 conditions the surfaces of metals receive permanent impressions from appro 

 priate objects placed upon them. Such facts show that the molecules of sub 

 stances propagate in all directions special ethereal undulations determined by 

 their special constitutions. 



