Life and Death, Heredity and Evolution 



often the number is not the same in the progeny, and the suc- 

 cessive progeny of the same parent may have different num- 

 bers of spines. As we pass from parent to offspring, similar 

 variations arise as to length of spines, number of teeth, and 

 size of the shell. These facts are illustrated in Figures 19 

 and 20. 



Now the question of interest is, whether these differences 

 within the single race (all derived by fission from a single 

 parent) show any tendency to be inherited. When a single 

 parent produces one offspring with few spines and another 

 with many spines, does the former tend then to produce a 

 set of progeny with few spines, the latter a set with many? 

 If so, we have the beginning of the origin of two races from 

 one. Or will there be mere chance variations, with no tend- 

 ency on the part of the later descendant to reproduce its 

 parent's peculiarities ? 



Study shows that there is certainly no complete or even 

 very marked tendency for the progeny within a race to 

 reproduce the diverse peculiarities of the parents. If a 

 parent has many spines, some of its offspring have many, 

 some few ; if the parent has few spines, some of its offspring 

 have many spines, some few ; and so of all the other charac- 

 ters (see Figures 20 and 21). To get any results whose 

 meaning is clear, we have to resort to averages, and to 

 mathematical measures of correspondence, for very large 

 numbers of parents and progeny belonging to a single race. 

 What we have to do is to determine whether, within a single 

 stock, on the average and in the long run, parents with many 

 spines produce offspring with a greater number of spines 

 than do parents with few spines. 



When we do this we find in Difflugia indications that there 

 is some correspondence between parents and progeny. In 

 some cases the indications are very slight; in others more 



