326 ENVIRONMENT AMONG ANIMALS AND PLANTS 



formation of the skeleton cannot occur ; the secretion of the 

 skeleton depends on the presence of some sulphate and alkalinity, 

 the skeleton requires calcium carbonate, cilia will only beat in 

 an alkaline medium containing potassium and magnesium, and 

 muscle will only contract when potassium and calcium are there.' l 



Summing up the results of these and other experiments, 

 Jenkinson says : ' Every factor, or nearly every factor [of the 

 environment], is necessary for this or that phase or part of the 

 process, some for the whole. Light of a certain wave length will 

 accelerate development ; light of another kind, or, in some 

 instances, darkness, will retard it, or will stop it altogether ; 

 a certain degree of heat is indispensable ; oxygen is required for 

 respiration, water for growth ; some eggs demand constant 

 agitation, others complete rest ; fertilization or segmentation 

 or gastrulation, or some one or other of the later phases of 

 development may depend absolutely on the presence of some 

 particular chemical element ; remove the factor in question, 

 whatever it may be, and that particular process will not occur, 

 and the specific typical end which is reached in normal develop- 

 ment will not be attained.' 2 



Putting aside until later the question as to what is implied 

 by the term normal environment, we may now ask what the 

 relation is between heredity and environment. To regard 

 a developing organism as subject to the action of two forces, 

 one tending to thrust it in one direction and the other in another, 

 is to view the matter in a wholly false light. Heredity and 

 environment are, as is clear from the evidence given above, 

 complementary one to the other ; without a germinal constitution 

 there can be no organism ; without appropriate stimuli an 

 organism cannot develop. Similarly it is misleading to speak 

 of the relative importance of the two factors unless the terms 

 are very clearly defined. To see that careful definition is necessary, 

 we have only to remember that, because, with the exception of 

 the anaerobic bacteria, all organisms require free oxygen, environ- 

 ment might be held to be all-important. Here, however, we are 

 not concerned with the problem as to the conditions under which 

 we might speak of the relative importance of the two factors ; we 

 have merely to note that they are essentially complementary. 



2. We may now observe that, just as a normal adult only 



1 Jenkinson, loc. cit., p. 161. 2 Ibid., p. 157. 



