72 HEREDITY AND VARIATION 



dwindles to a vanishing point. It is characteristic of organisms 

 to pass through a series of cychc changes. 



3. Changes inYolyed in Functioning. — ^As contrasted with 

 inanimate systems, organisms are characterised by their power 

 of elective response to environmental stimuli. A living creature's 

 responses tend towards self-preservation or species-preservation. 

 Though they may fail, the reactions are primarily and funda- 

 mentally effective. And these functionings or effective responses 

 necessarily involve changes in the system. They involve wear 

 and tear, and leave more or less discernible results. Normally, 

 however, the results, known as fatigue-effects and the like, are 

 obliterated by nutrition, rest, and other forms of recuperation. 

 In the study of an intricate structure, like a bee's brain, it is 

 possible to arrange on an inclined plane the changes which 

 are normally obliterated by a night's rest, the changes which 

 require prolonged recuperation before they disappear, and the 

 changes which cannot be recovered from — which accumulate 

 until the bee dies a natural death. 



4. Temporary and Indi¥idual Adjustments. — In addition 

 to the inherent primary power of effective response, organisms 

 have different degrees of plasticity. They can adjust their 

 reactions to novel conditions. They can " try " first one mode 

 of reaction and then another, finally persisting in that which 

 is most effective. Even the unicellular Infusorians do this. 

 How much of this plasticity is primary, or inherent in the very 

 nature of living matter, how much of it is secondary and wrought 

 out by Natural Selection in the course of ages, must remain in 

 great measure a matter of uncertainty. Each case must be judged 

 on its own merits. It is certain that many unicellular organisms 

 are very plastic, and it seems reasonable to suppose that as 

 differentiation increased, restrictions were placed on the primary 

 plasticity, while a more specialised secondary plasticity was 

 gained in many cases, where the organisms lived in environments 

 liable to frequent vicissitudes. It' is convenient to use the 



