THEORY OF EVOLUTION 



171 



method does not claim to prove that modifiers 

 are ]3resent, but it shows why certain results 

 are in harmony M'ith that exi^ectation 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 ra])])its 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 tliat the varia])ility was smaller in 

 the first generation than in the second genera- 



Classifiration in relation to jtnrrntx l>ast'(l on sknil Ifnc/tJis and ulna 

 lenf/Uii, to s?ioir the relatirp r-arialjilitf/ of tiro measurements and 

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



Sit 'lie fa Vt- cnit'iniieil 



CHARACTER 



GENE- 

 RATION 



Length of f 

 skull 1 



Length of j 

 ulna I 



BO. 

 Fi 



B.n. 



Q O 



3S341f 



111 



lSl.il- 



\ 

 SllO'll 



I I 

 121311415:161 17118 



510 7 3 2 2' 

 8 4 3 1 

 5 3 17 3 



lai.ill 5 2 J 2 2 



1 



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