180 MUTATION 



of domesticated organisms behave in the same 

 way, as illustrated by poultry. Unit characters, 

 then, in so far as they refuse to blend, will not 

 be swamped by intercrossing, but will reappear 

 intact in a predictable proportion in successive 

 generations. 



The second difficulty which the mutation doc- 

 trine solves is discontinuity between species. 

 Species differ in the presence or absence of cer- 

 tain unit characters. These unit characters are 

 typically discontinuous in their origin. Hence 

 it is futile to look for intergrades ; as well might 

 one look for intergrades between carbon monox- 

 ide and carbon dioxide. Species are discontin- 

 uous because specific characters are discontin- 

 uous ; and specific characters, in so far as they are 

 unit characters, are discontinuous because the 

 molecular changes upon which they depend are 

 discontinuous. 



It is rash at the threshold of any. new science 

 to accept any one hypothesis to the exclusion of 

 others. The president of our Association has 

 taught us our duty toward multiple hypotheses. 

 As in the newer chemistry transitions between 

 molecules are becoming a recognized possibility, 

 so it can not be denied that some unit characters 

 may arise gradually; or, as a result of repeated 

 crossing, show true blending and intergrading 

 conditions. Many characters are indeed less or 

 more because they have an ontogeny, and the 

 adults stop at different points in the ontogeny. 



