92 A Factor in Evolution 



ing by 'use and disuse' from these new functions, our 

 further question is about them. And the question is this : 

 How does an organism come to be modified during its life 

 history ? 



In answer to this question we find that there are three 

 different sorts of ontogenic agencies which should be dis- 

 tinguished each of which works to produce ontogenetic 

 modifications or accommodations. These are : first, the 

 physical agencies and influences in the environment which 

 work upon the organism to produce modifications of its 

 form and functions. They include all chemical agents, 

 strains, contacts, hinderances to growth, temperature 

 changes, etc. So far as these forces work changes in 

 the organism, the changes may be considered largely 

 'fortuitous ' or accidental. 1 Considering the nature of the 

 forces which produce them, I propose to call these modifi- 

 cations 'physico-genetic.' Spencer's theory of ontogenetic 

 development rests largely upon the occurrence of lucky 

 movements brought out by such accidental influences. 



Second, there is a class of modifications, in addition 

 to those mentioned, which arise from the spontaneous 

 activities of the organism itself in the carrying out of its 

 normal life-functions. These modifications and adjust- 

 ments are seen to a remarkable extent in plants, in uni- 

 cellular creatures, in very young children. There seem 

 to be a readiness and a capacity on the part of the organ- 

 ism to 'rise to the occasion,' as it were, and make gain 

 out of the circumstances of its life. The facts have been 

 put in evidence (for plants) by Henslow, Pfeffer, Sachs ; 

 (for micro-organisms) by Binet, Bunge ; (in human pathol- 



1 That is, so far as any direct provision for them is found in the economy 

 of the organism's growth. 



