THE WONDER OF LIFE 511 



respond to stimuli, but often self-destructively, whereas 

 the living creature's responses tend to self-preservation 

 or to species-preservation. Not that the organism can 

 respond successfully to all stimuli, for instance to a strong 

 current of electricity, for it is not able to live anywhere or 

 anyhow, but only within certain environmental limits. 

 We cannot account for this primary and fundamental 

 power of effective response ; it is part of our conception 

 of life. There could have been no organisms at all unless 

 they had possessed something of this power of answering 

 back and yet retaining their integrity. In some degree it 

 must have been part and parcel of the first and 

 simplest organisms, and it has been improved upon ever 

 since. 



(2) Plasticity. Another important fact is that living 

 creatures are in different degrees plastic. That is to say, 

 they can adjust their reactions to novel conditions, they 

 are not rigidly stereotyped in their responses. In many 

 cases, even among the simplest organisms, the animal that 

 is up against a difficulty, ' tries ' one mode of reaction 

 after another, and may eventually find one which is effective. 

 Professor Jennings reports that the behaviour of certain 

 Infusorians may be compared to a pursuance of ' the 

 method of trial and error '. There are not a few cases of 

 marine animals showing sufficient plasticity to adjust 

 themselves in their own individual lifetime to the very 

 different conditions of fresh water. We see plasticity 

 too when animals are transported from one habitat to 

 another where different habits are required. It is convenient 

 to use the term ' accommodation ' for the frequently occur- 

 ring functional adjustments which organisms are able to 

 make to new conditions. Thus there may be an interest- 



