EVOLUTIONAL ZOOLOGY 227 



may be shown by breeding from the individuals separately 

 and observing the range of fluctuating variability exhibited by 

 the offspring of the isolated beans. When this is done, it is 

 found that any bean belonging to a given pure-line, whether 

 it be a light or a heavy one or one of intermediate weight 

 within the limits of the pure-line, will reproduce beans fluc- 

 tuating in weight between these limits, but it cannot reproduce 

 the series of weights embraced within the limits of the entire 

 mixed population. In other words, it cannot transcend the 

 limits of the strain to which it belongs, and after a pure-line 

 is once segregated from the rest of the population, further 

 selection becomes useless. We, therefore, can neither increase 

 nor decrease the average weight of beans derived by breeding 

 from individuals which have been selected from isolated pure- 

 lines. Any further increase or decrease will depend solely 

 upon the appearance of a mutation and the establishment of 

 a new pure-line. 



GENETICS OR THE PHYSIOLOGY OF HEREDITY 



I have emphasized throughout this lecture that organic 

 evolution is a process whose essential aspects are variation 

 and heredity. If variations did not occur, the offspring would 

 be replicas of their parents, and even though differences did 

 appear in the individual, if they were not transmitted, it would 

 still be of no avail, for the life of the new form would then 

 be for a generation only. At bottom, therefore, the problem 

 of evolution is two-fold — the problem of the causes of heri- 

 table variations, and the problem of elucidating the laws gov- 

 erning hereditary transmission. In reality these are but dif- 

 ferent aspects of the same fundamental problem, the problem 

 which the science of genetics has set itself to solve. 



While zooloogists in the early years of the Darwinian 

 period were absorbed in their evolutionary speculations and 

 the spinning of ancestral relationships, an Austrian monk, 



