252 ESSENTIALS OF BIOLOGY 



greatly from those under which their wild progenitors lived. In 

 short, there is some evidence that marked changes in the environ- 

 mental conditions — which changes do not render organisms 

 " unhealthy," or inhibit their reproductive powers — may affect 

 the gonads and the developmental processes. It is not known 

 whether the germ-cells are thus affected directly, or via the 

 general bodily tissues that environ them. It was inconceivable 

 to Weismann that the germ-plasm (that is, the chromosomal 

 material of the germ-cells) could so be affected, but there seems 

 now to be no doubt that this may happen. Now such reaction 

 between the germ-cells and the environment is something 

 analogous to the purely physical processes alluded to above. 



But a mutation is a change in some character of an organism 

 which is also displayed by the progeny, and the progeny of the 

 progeny, etc., of that organism : it is said to be an inheritable 

 change. Therefore the developmental organization has been 

 changed. We note, in passing, a fact of much significance, that 

 the mutation " Mendelizes," that is, it may appear in some of 

 the progeny, or grand-progeny, etc., but not in others. When 

 it does not appear in the progeny, but reappears in the grand- 

 progeny it is said to be " recessive " — it is still there, in a way, 

 but is not manifested. This is the fact of observation which must 

 clearly be distinguished from the statements of the hypothesis 

 of genes. We have already noted it in saying that the agencies 

 which lead to the appearances of the mutational character are 

 *' loosely attached " to the developmental organization. Now it 

 is clear that the latter is not a physico-chemical system, or, at least, 

 we have seen what are the enormous difficulties in believing it 

 to be such. Therefore it is not easy to see how an agency which 

 is best conceived as psychological in nature can react to some 

 physico-chemical change in the environment in the way that, 

 say, yellow phosphorus reacts when it changes to the red, allotropic 

 form. We can make analogies : thus builders accustomed to 

 work with stone are compelled to work with steel and concrete 

 so that the designs of buildings have undergone " mutations." 

 But obviously the analogy is indicative at the best and it is suspect, 

 to many minds, because it is '* anthropomorphic." Clearly the 

 problem of the *' origin of mutations " is not yet satisfactorily 

 dealt with. Perhaps it is a pseudo-problem that arises from our 

 difficulty in postulating changes to happen without those changes 



