Jennings was able by means of selection to get from the descendants of one original individual a number of different types that themselves bred true, except in so far as selection could affect another change in them. In this connection it is interesting to note that Leidy
has published figures of Difflugia (fig. 92) that show that a great many "types" exist. If through sexual union (a process that occurs in Difflugia) the germ plasm (chromatin) of these wild types has in times past been recombined, then selection would be expected to separate certain types again, if, at division, irregular sampling of the germ plasm takes place. Until these points are settled the bearing of these important experiments of Jennings on the general problem of selection is uncertain.
How Does Natural Selection Influence the Course of Evolution?
The question still remains: Does selection play any rôle in evolution, and, if so, in what sense? Does the elimination of the unfit influence the course of evolution, except in the negative sense of leaving more room for the fit? There is something further to be said in this connection, although opinions may differ as to whether the following interpretation of the term "natural selection" is the only possible one.
Fig. 93. Evolution of elephant's skulls. (After Dendy.)
If through a mutation a character appears that is neither advantageous nor disadvantageous, but indifferent, the chance that it may become established in the race is extremely small, although by good luck such a thing may occur rarely. It makes no difference whether the character in question is a dominant or a
recessive one, the chance of its becoming established is exactly the same. If through a mutation a character appears that has an injurious effect, however slight this may be, it has practically no chance of becoming established.
