THEORY OF EVOLUTION 



171 



method does not claim to prove that modifiers 

 are present, but it shows why certain results 

 are in harmony with that expectation and can 

 not be accounted for on the basis that a factor 

 has changed. Let me give an example. When 

 a Belgian hare with large body was crossed to 

 a common rabbit with a small body the hybrid 

 was intermediate in size. When the hybrid 

 was crossed back to the smaller type it pro- 

 duced rabbits of various sizes in apparently 

 a continuous series. MacDowell made meas- 

 urements of the range of variability in the first 

 and in the second generations. 



He found that the variability was smaller in 

 the first generation than in the second genera- 



Classification in relation to parents based on skull lengths and ulna 

 lengtJis, to show the relative variability of two measurements and 

 of the first generation (F\) and the back cross (B. C.) 



same ta'<li' continued 



