EVOLUTION TODAY 433 



chromosomal complex of a species and the genes which are con- 

 tained therein. The cj'toplasm of somatic cells is the structural 

 material in which the differentiation of tissues is wrought and 

 some biologists have questioned whether the genes might continue 

 to exert an effect on this substance throughout life. ExiDerimental 

 methods for answering such a query have not been devised, l)ut 

 the only logical opinion possible in the light of biological facts 

 is that they do remain active even in somatic cells, for living 

 substance is constantly being torn down and built up through 

 the processes of metabolism, and in enucleate cells constructive 

 metabolism ceases. 



It is logical to conclude that the genes which are responsible 

 for the initial production of characters in the developing individual 

 are no less responsible for all of the characters which appear during 

 its life. It is the gene, in the end, which responds to a definite 

 environmental stimulus for the production of a definite character. 



The Source of Change. When a certain result can be secured 

 by the proper association of materials and forces we find that an 

 accurate formula is essential to accurate results. The cake turns 

 out well if the proper ingredients are combined in the proper way 

 and baked at the right temperature. Iron and steel take a definite 

 temper if heated to a definite degree and properly quenched. But 

 vary one of these factors and the cake is ruined or the metal does 

 not have the desired quality. If the association of definite factors 

 produces a definite result, it is to be expected that different factors 

 would produce different results. 



There is no reason to suppose that living matter is different in 

 relation to these fundamental truths. Every character in a living 

 organism is due to definite qualities of the heritage, reacting to 

 definite stimuli received from the environment. The two are no 

 more separable than batter and heat in the baking of a cake. 

 We must therefore recognize that change in living things cannot 

 fail to result from a change in either of the components. 



Scientists have been prone to recognize spontaneous change in 

 the heritage. It would be difficult to prove the truth of such an 

 opinion, for we have seen that living matter is active only in 

 response to environmental stimuh. Moreover, in the analogies 

 cited, change of substance is due to outer forces — either human 

 interference or forces in the physical world. The environment, 

 however, varies according to a vast complex of physical forces 



