598 Mutations 



We are trained to the appreciation of the dif- 

 ferentiating marks of systematic species. When 

 we have succeeded in discerning these as given 

 by our local flora lists, we rest content. Meet- 

 ing them again we are in the habit of greeting 

 them with their proper names. Such is the 

 satisfaction ensuing from this knowledge that 

 we do not feel any inclination for further in- 

 quiry. Striking deviations, such as many 

 varietal characters, may be remarked, but then 

 they are considered as being of only secondary 

 interest. Our minds are turned from the deli- 

 cately shaded features which differentiate ele- 

 mentary species. 



Even in the native field of the evening-prim- 

 roses, no botanist would have discovered the 

 rosettes with smaller or paler leaves, constitut- 

 ing the first signs of the new species. Only by 

 the guidance of a distinct theoretical idea were 

 they discovered, and having once been pointed 

 out a closer inspection soon disclosed their 

 number. 



Variability seems to us to be very general, but 

 very limited. The limits however, are distinct- 

 ly drawn by the struggle for existence. Of 

 course the chance for useful mutations is a very 

 small one. We have seen that the same muta- 

 tions are as a rule repeated from time to time 

 by the same species. Now, if a useful mutation, 



