62 THE CONTROL OF LIFE 



activities, habits, use and disuse, and so on. These 

 ' modifications,' as they are now technically called, are 

 structural changes that persist after the inducing con- 

 ditions have ceased to operate. Thus a lifelong tanning 

 of the skin, a callosity on a much pressed part of the skin, 

 a strengthening of a muscle by use, a degeneration of a 

 muscle by disuse, an accumulation of fat as the result 

 of gluttony, a strain of the eyes through overwork, may 

 serve to illustrate ' modifications.' They are often very 

 important for the individual, both for good and ill, 

 but they do not seem of much, if any, direct racial 

 importance, since the evidence of their transmission is 

 rare or unsatisfactory, or emphatically absent. They 

 are to be thought of as dints impressed from without, 

 and contrasted with outcomes expressed from within. 

 The latter are called variations, mutations, or new depar- 

 tures, and are often highly transmissible. They result 

 from changefulness inherent in the germ-cells, provoked 

 into expression, it may be, by penetrating stimuli 

 from without, and they form the raw material of organic 

 evolution. Whether we believe in the transmissibility 

 of * dints ' or not, we know that they are not commonly 

 transmitted in any measurable manner. We also know 

 that it is of practical as well as theoretical importance 

 to distinguish what is due to peculiarities of ' nurture ' 

 from what is the expression of inborn ' nature.' 



§5. Different Modes of Inheritance 

 (a) In starting a breed of domesticated animals, 

 such as Polled Angus cattle or Ancon sheep, the breeder 



