THE FACTOES OF ORGANIC EVOLUTION. 73 



effected. As soon as these became decided, natural selec 

 tion came into play; for inevitably the uiilikenesses 

 produced among the units had effects on their lives : there 

 was survival of some among the modified forms rather 

 than others. Utterly in the dark though we are respect 

 ing the causes which set up that process of fission 

 everywhere occurring among the minutest forms of life, 

 we must infer that, when established, it furthered the 

 spread of those which were most favourably differentiated 

 by the medium. Though natural selection must have 

 become increasingly active when once it had got a start ; 

 yet the differentiating action of the medium never ceased 

 to be a co-operator in the development of these first 

 animals and plants. Again taking the lead as there arose 

 the composite forms of animals and plants, and again 

 losing the lead with that advancing differentiation of 

 these higher types which gave more scope to natural 

 selection, it nevertheless continued, and must ever con 

 tinue, to be a cause, both direct and indirect, of 

 modifications in structure. 



Along with that remarkable process which, beginning in 

 minute forms with what is called conjugation, developed 

 into sexual generation, there came into play causes of 

 frequent and marked fortuitous variations. The mixtures 

 of constitutional proclivities made more or less unlike by 

 unlikenesses of physical conditions, inevitably led to occa 

 sional concurrences of forces producing deviations of 

 structure. These were of course mostly suppressed, but 

 sometimes increased, by survival of the fittest. When, along 

 with the growing multiplication in forms of life, conflict 

 and competition became continually more active, fortuitous 

 variations of structure of no account in the converse with 

 the medium, became of much account in the struggle with 

 enemies and competitors ; and natural selection of such 

 variations became the predominant factor. Especially 

 throughout the plant- world its action appears to have 



