102 THE BEGINNINGS OF LIFE. 



that a perusal of Appendix D (in which we have 

 given an account of some of the most remarkable 

 and well-attested variations of these lower forms) may 

 perhaps bring more prominently before many of our 

 readers the full extent of this plasticity than if they 

 were to read mere isolated statements. We have seen 

 how the structure and nature of certain crystals may be 

 wholly changed by some modification in the conditions 

 to which they are subjected. We have learned, in fact, 

 how potent, within certain limits, is the influence of 

 the forces of the environment, in comparison with that 

 comprised under the head of ^ inherent tendencies,' in 

 determining crystalline form ; so that if a similar 

 characteristic should be displayed by new-born living 

 matter, or by simple organisms, this would be merely a 

 verification of what might have been predicted a priori. 



We have now frequently stated that the essential 

 characteristic of living matter is its extreme plasticity, 

 and its power of carrying on a reciprocal series of 

 changes and rearrangements amongst its constituent 

 molecules in response to changes in its medium or 

 environment. Its history is one of continual flux and 

 change. And seeing that those causes which operate 

 in checking the modifying influence of external con- 

 ditions in complex and more completely individual- 

 ized organisms, do not come into play in the most 

 elementary organisms or in others which are made up 

 to a large extent by a repetition of similar parts, we 

 might expect that the latter would be prone to undergo 



