50 Life and Death, Heredity and Evolution 



This problem of the origin of inherited variations, and of 

 the nature and grounds of evolution, meets us in these 

 animals in a peculiarly simple form. For in their repro- 

 duction from a single parent it is not complicated by the 

 continual mixing of diverse lines of descent, which enormous- 

 ly confuses the matter in higher organisms. In creatures 

 in which reproduction is always from two parents, the de- 

 scent of generations takes the form of a network, such as 

 illustrated at B in Figure 5. If we attempt to trace back 

 the ancestry of any one of the individuals indicated by the 

 lines at the right, we find that it is a mixture of many lines 

 of descent, with diverse hereditary characters ; in any given 

 past generation many ancestors of the present stock appear. 

 It becomes extremely difficult or impossible to predict what 

 hereditary characters it should show, or whence it has de- 

 rived those that do appear; and it is hardly possible to 

 distinguish an actually new character from one resulting 

 from the mixture of earlier stocks. In the Protozoa while 

 descent from a single parent is in progress, the passage of 

 generations takes the form indicated at A in Figure 5. 

 There is no mixing of diverse lines, and in any past genera- 

 tion there is but one ancestor for the existing stock ; descent 

 can be traced backward in a single line. On account of this 

 relatively simple state of affairs, the origin of variations and 

 the course of evolution has been much studied in the Proto- 

 zoa. We will illustrate the conditions found by means of a 

 series of figures of Difflugia corona (Figures 19-21). 



When we examine a single species, be it a bacterium, a 

 rhizopod (as Difflugia corona) or a ciliate infusorian (as 

 Paramecium aurelia), we find a great diversity in the in- 

 dividuals, in their form, their structure (Figure 19), and 

 their physiology. Part of this diversity is, in some of these 

 creatures, due to the different conditions under which they 

 are living. But if we bring them all into the same conditions 



