FACTORS OF INHERITANCE. 55 



due to man having lost certain factors which this early 

 ancestor possessed, or to the factors being shuffled. At 

 some period in evolution new factors must have been in- 

 troduced into the inheritance, and the process is presum- 

 ably still going on. 



Speculating as to the possible origin of mutations we 

 are inevitably led to the conclusion that any transmis- 

 sible differences arising between two originally identical 

 germinal constitutions formed of the same factors of in- 

 heritance must come from the environment ; for any 

 closed system must reach a state of equilibrium, and con- 

 tinue unchanged unless influenced from without. Muta- 

 tions, then, are produced by the addition to, subtraction 

 from, or alteration in, the factors of inheritance already 

 present. 



This whole system of factors may appear to the reader 

 somewhat artificial ; yet it is the legitimate deduction 

 from facts for which no other explanation can at present 

 be given. What the factors are we can only surmise. 

 They are not distinct corpuscles of living matter like the 

 hypothetical gemmules of Darwin or biophors of Weis- 

 mann, representing separate cells or parts of the organ- 

 ism. Rather should they be conceived as definite 

 chemical substances forming part of, or entering into, 

 the chain of metabolising compounds and thereby in- 

 fluencing the course of metabolism in such a way as to 

 bring about a certain definite result. 



If such a substance is destroyed, or fails to pass into 

 the gamete, a retrogressive mutation arises. If the 

 physico-chemical structure of a factor is altered by the 

 shifting and rearrangement of the chemical atoms, for 

 instance, a mutation may result. A truly progressive 

 mutation would be produced when new atoms or com- 

 pounds become permanently involved in the metabolic 

 cycle of the germ-plasm. 



One must be careful not to assume that because a 

 character appears to vary independently it is necessarily 



