Influence of Environment upon the Organism. 461 



9. Degrees of Environmental Influence. — It is obvious, from 

 the illustrations which have been given, that the environment 

 may condition changes varying greatly in their degree of 

 directness. If the environment remain constant, it is assumed 

 that it will preserve the organism in stoM quo. If it change, 

 there may be the following different modes of action : — 



(1.) When an Amoeba shrinks up under a sudden beam of 

 light; when a drop of dilute ammonium carbonate brings 

 down a sun-animalcule from its normal heliozoon form to an 

 amoeboid phase ; when other influences reduce ciliated cells, 

 or even sperms, to the amoeboid level ; when the Infusorian 

 pulsates violently in deficient oxygen ; when heat increases 

 their locomotion ; change within the organism may be said to 

 follow as the direct result of change in external conditions. 

 The simplest cases of all are those in which a surplus of 

 some form of energy, such as heat or food, is received by the 

 organism, and forthwith expressed in some probably equivalent 

 organismal change. In higher organisms, and with more 

 complex environment, though we can still say that altered 

 conditions have been followed by a certain change, it is, as 

 yet, impossible, in the majority of cases, to form any concep- 

 tion of the series of changes from the original external one to 

 the final internal result, impossible also to say how far the 

 internal modification is proportionate to the external change, 

 or how far it is the result of a series of changes to which the 

 new stimulus simply gave the initial impulse, as the pulling 

 of a trigger to the discharge of a rifle. But still it seems 

 legitimate to regard a large number of instances as internal 

 variations directly conditioned by external changes. 



(2.) All who have attacked the problem of variation have 

 allowed that one variation may bring another in its train. 

 A change in respiration may bring about a change in circula- 

 tion ; a change in the nervous system may be followed by a 

 change in the muscular. And thus a second category may 

 be established for secondary or correlated internal variations, 

 consequent on primary environmentally conditioned changes. 



(3.) When the organism immediately responds to a change 

 in outside conditions, its action may, in many cases, be 

 described as a direct parry \ and in some cases this appears 



