80 Life and Death, Heredity and Evolution 



Figure 22, showing the diverse races of Paramecium, we see 

 that in any single race there are individuals of many different 

 sizes. But almost all these differences are matters of age, 

 nutrition, and the like. So when we select and separate 

 large and small individuals, we are likely to get merely well- 

 grown, well-fed individuals in one set; young, ill-nourished 

 ones in the other. Even if there are arising really hereditary 

 differences in size, we cannot distinguish these from the much 

 more numerous transitory changes, so that our process of 

 selection may be rendered quite without hereditary effect. 

 Similar difficulties beset any attempt to select for other 

 characters that are dependent on growth and present en- 

 vironmental conditions. 



It appears possible therefore that the difference between 

 the results of selection in Difflugia and in other organisms is 

 due to these facts ; that there is no real difference as to the 

 sort of thing that happens, but only as to whether one can 

 detect the hereditary changes that actually occur. Are we 

 to believe that the hereditary constitution of parent and 

 progeny is actually identical in these other forms? Or shall 

 we find changes in it, if we study with sufficient minuteness? 



Now on this point we have a certain amount of evidence, 

 based again on what I have ventured to call our "second 

 degree" investigation of these matters. If we watch the 

 divisions of Paramecium or of any of its relatives, we find 

 that the two individuals produced by the division of one 

 do not always behave exactly alike ; after they have grown 

 to adult size, one of them often divides before the other 

 does (see Figure 25). Are such differences due to some 

 change in the fundamental and hereditary constitution, or 

 only to some slight difference in nutrition or the like ? Here 

 was an opportunity for minute study of the matter; it was 

 undertaken in our laboratory by Middleton (1915). He 



