70 Life and Death, Heredity and Evolution 



all sources of confusion met in the earlier studies, limit our- 

 selves to one single race, pursue its history with more minute 

 and unwearying steadiness, for longer periods than has 

 before been done, to such a degree that we may properly 

 call our studies investigations in the second degree, as 

 compared with the earlier ones ? 



This is what I decided four years ago to attempt ; with a 

 number of associates to set on foot such "second degree" 

 investigations. They have now been carried far enough to 

 show results. The main difficulties in the previous work 

 along these lines have been the following: 



(1) In these simple organisms it is difficult to find 

 definite distinctive characters, such as color of eyes or hair, 

 that are inherited. In Paramecium, for example, all the in- 

 dividuals are very similar, the diversities being mainly slight 

 differences in size or shape; or in indefinite physiological 

 traits. Such characters are not favorable for work in 

 heredity because they are hard to distinguish; yet in prac- 

 tically all the earlier work such characters were employed, 

 for the good reason that these seemed the only characters 

 available. 



(2) Further, the characters studied have been such as 

 were continually changed by growth during the life of the 

 animals ; and likewise greatly modified by changes in environ- 

 mental conditions. 



The first thing to do therefore was to find if possible an 

 organism in which these difficulties did not exist. From this 

 point of view an ideal animal was found in Difflugia corona, 

 the creature that I have already employed in these lec- 

 tures to illustrate a number of points. This animal is an 

 amceba with a shell, and the shell presents a number of 

 definite characters that can be easily counted and measured 

 (Figure 23). These characters are: the number of spines; 



