CONGRUITY OF THE EVIDENCE 293 



the ultimate units, the "unknown somethings which have the 

 power of organising themselves into a structure of this or that 

 species." ' I merely contend, therefore, that these ultimate units, 

 looking to their nature and origin, must be supposed to be capable 

 of undergoing isomeric changes within certain limits, and that, 

 such changes having been brought about, these altered units would 

 tend to unfold into new forms — more or less different from the 

 parent forms, and corresponding with this or that kind of hetero- 

 genetic transformation. The number of such transformations now 

 known is considerable among lower organisms, though compara- 

 tively rare among higher plants and animals — where they are 

 represented by sports and the ' Mutations ' of de Vries. 



Great differences exist with respect to the degree of variation 

 that may be induced in different lower organisms, within similar 

 periods, under the influence of any given changes in their en- 

 vironments. Changes, which may be almost inoperative in pro- 

 ducing a modification of some organisms, may produce profound 

 alterations in others. And, similarly, while a very prolonged 

 continuance of altered conditions is needful to affect some organ- 

 isms, the influence of changed conditions, on others, may be rapid 

 and more or less immediate. 



The greater the differentiation and complexity of any organism, 

 the less is it likely to be influenced by slight or temporary modifi- 

 cations in the ' conditions ' or influences to which it is subjected. 

 The complexity has been gradually attained, and each part or 

 organ has functions and structures which are definitely related to 

 the functions and structures of other parts. The whole is com- 

 posed of parts working in accord with one another, and in such a 

 manner as to establish a harmony between the actions going on 

 within and those without the organism. The result of this inter- 

 action during past time has been the gradual elaboration of an 

 organism of a certain structure ; so that this structure and form 

 are only to be regarded as the physical expressions of an approxi- 

 mate moving equilibrium between numerous related factors — 

 between the inherent tendencies of ' physiological units ' under the 

 influence of all past ' conditions,' and the present operation of 

 external forces upon the now-acquired structure. When, therefore, 

 unaccustomed conditions act upon such organisms, they are unable 

 easily, or within short periods, to produce direct modifications of 



' hoc. cit, p. 370. 



