40 THE FACTORS OP ORGANIC EVOLUTION. 



of certain forces on certain materials ; and when it dwindles, 

 there is either a lack of some materials, or the forces co 

 operate in a way different from that which produces growth. 

 If a structure has varied, the implication is that the processes 

 which built it up were made unlike the parallel processes 

 in other cases, by the greater or less amount of some one or 

 more of the matters or actions concerned. Where there 

 is unusual fertility, the play of vital activities is thereby 

 shown to have deviated from the ordinary play of vital 

 activities ; and conversely, if there is infertility. If the 

 germs, or ova, or seed, or offspring partially developed, 

 survive more or survive less, it is either because their 

 molar or molecular structures are unlike the average ones, 

 or because they are affected in unlike ways by surrounding 

 agencies. When life is prolonged, the fact implies that 

 the combination of actions, visible and invisible, consti 

 tuting life, retains its equilibrium longer than usual in 

 presence of environing forces which tend to destroy its 

 equilibrium. That is to say, growth, variation, survival, 

 death, if they are to be reduced to the forms in which 

 physical science can recognize them, must be expressed 

 as effects of agencies definitely conceived mechanical 

 forces, light, heat, chemical affinity, &c. 



This general conclusion brings with it the thought that 

 the phrases employed in discussing organic evolution, 

 though convenient and indeed needful, are liable to mislead 

 us by veiling the actual agencies. That which really goes 

 on in every organism is the working together of component 

 parts in ways conducing to the continuance of their com 

 bined actions, in presence of things and actions outside ; 

 some of which tend to subserve, and others to destroy, the 

 combination. The matters and forces in these two groups, 

 are the sole causes properly so called. The words &quot;natu 

 ral selection,&quot; do not express a cause in thefphysical sense/1 

 They express a mode of co-operation among causes or 

 rather, to speak strictly, they express an effect of this&amp;gt; 



