MUTATIONS 1 1 1 



and a new race is established on the earth. This new group 

 and its center constitute a mutation, and the individuals are 

 spoken of as mutants. Variation has become discontinuous as 

 well as continuous. 



These sudden offshoots from established species were noted 

 by Darwin, who called them sports. He considered, however, 

 that new species are formed only by the slow but continuous 

 action of selection working with ordinary fluctuations (continuous 

 variations), building up new types a little at a time through the 

 gradual accumulation of slight but favorable deviations. 



His so-called " sports " were therefore mysterious, and from 

 the fact that under natural conditions they generally disappear 

 rapidly by crossing, he was led to attach little importance to 

 these sudden departures from the established type. 1 Later 

 researches, however, have given them unexpected significance. 



The distinguishing feature of a mutation is that there are no 

 intermediates between the old type and the new, which was 

 therefore attained not by slow degrees but by a sudden leap ; 

 that there is but a slight tendency to revert to the old form, 

 but that if reversion takes place at all it is complete at once and 

 the return is to the old type and not to an intermediate form. 

 The mutant is distinctly a case of discontinuity. 



SECTION II EXAMPLES OF MUTATION 



The classic examples of mutation are the weeping willow and 

 the nectarine. They are to be regarded, however, as familiar 

 illustrations of general principles widely operative and giving 

 rise not to few but to many distinct types. 



When a seed germinates it puts out two sprouts. One is 

 positively geotropic ; that is, it responds to the force of gravity 

 and grows downward into the soil, developing the root system. 

 The other is negatively geotropic ; that is, it grows upward 



1 The student of evolution should gain the conception that the type of a race 

 is not a fixed point from which deviations radiate ; it is rather the center of 

 gravity of all the individuals of the race, its exact location depending upon the 

 extent and direction of individual deviations, shifting slightly from time to time 

 with the causes that influence variability. 



