THE 



AMERICAN NATURALIST 



Vol. XLVII December, 1918 No. 564 



THE FIXATION OF CHARACTER IN ORGANISMS 1 

 DR. EDMUND W. SINNOTT 

 Harvard Uxiversity 



The segregation of animals and plants into those 

 groups which we call species, genera and families and the 

 arrangement of such groups in a natural system of classi- 

 fication are made possible by the fact that "during the evo- 

 lution of any group there 1 are always characters which 

 have varied comparatively little and, from their constancy 

 throughout large numbers of otherwise different individ- 

 uals, are therefore of great value in determining rela- 

 tionships. Should all the characters of an individual be 

 equally subject to change in the passage from one genera- 

 tion to another such chaos would result that anything but 

 the most arbitrary classification would be quite impos- 

 sible. It is of great importance to the taxonomist, the 

 experimental morphologist and the student of evolution 

 in general to ascertain, if possible, what are the causes for 

 these differences in degree of variability and to attempt 

 a formulation of the laws under which they appear. 



The first attempt at a scientific explanation of this 

 problem was put forward by the theory of Natural Selec- 

 tion. In its extreme form this theory assumes that all 

 conservative characters, known to be very ancient because 

 of their occurrence throughout large groups of organ- 

 isms, are characters of supreme importance in the 



