SELECTION WITHOUT AKTIFICIAL CKOSSING. 



27 



publications. First, he studied abnormal individuals to see whether 

 or not their abnormalities were inherited. In many of his cultures, 

 individuals could be found that had various peculiarities. Although 

 he sought patiently for some peculiarity of this kind that might be 

 inherited and although he found many such pecuharities, in no case 

 did he find one that is inherited in the proper sense of the word. 

 The peculiarities in question never appeared in both of the individ- 

 uals resulting from a division. Thus there was no tendency for 

 them to multiply and spread over the race. Concluding his discus- 

 sion of such cases, Jennings remarks: ''Examination of a large num- 

 ber of cases in Paramecium shows that these untypical characters 

 are never reproduced in the young." 



Jennings also found that the descendants of a single individual 

 varied greatly in size. This suggested the idea that by selecting 

 continually from the largest and the smallest, two races could be 

 developed which would differ in size, although descended from the 

 same original individual. This experiment was carried on for a 

 very long series of generations, one line consisting of the largest 

 individuals that could be found and of their largest progeny, the 

 other of the smallest individuals and their smallest progeny. At 

 the end of the experiment the two lines were brought under the 

 same environmental conditions and within a very short time the 

 average length of the two types became identical. This led Pro- 

 fessor Jennings to remark that ''Selection within a pure race is of 

 no effect on size," and again, "Neither selection nor environmental 

 action changes the size of the pure race." 



This investigator found eight distinct types of Paramecium in a 

 group which was previously supposed to consist of two species differ- 

 ing in size. Figure 1 illustrates the relative sizes of the individuals 

 in these eight races. It is seen that even the extreme forms overlap, 

 and it was found that if the smallest individual of the largest race be 

 selected and its progeny grown with continual selection from the 

 smallest individuals to be found, no matter how long such selection 

 was continued the progeny of this small individual would soon cover 

 the whole gamut of variation of the race to which it belonged, and 

 the same was true for each of the other races. Speaking of the 

 effect of selection on such a species, Jennings says: 



How will selection act on such a complex species? As we have seen, selection 

 within a single race is without effect. But if we make selections among the indi\'iduals 

 of a mixed collection of races, such as figure 1 shows, we reach most instructive results. 

 By making our selections in the proper way, we for a time make steady progress 

 toward a certain goal. We will suppose that we do not know of the existence of these 

 races; this is the case with most experiments in selection. From the species as a 

 whole, as shown in figure 1, we will select for increased size. Let us follow the old 

 plan of selecting many individuals showing the desired character; we will preserve 

 all specimens above the mean size of the entire collection; that is, we divide the 

 165 



