SELECTION WITHOUT AKTIFICIAL CROSSING. 



29 



Similar work with identically the same result has been done on 

 hydra by Elise Hanel^ and byM. A. Barber^ on yeasts and bacteria. 

 In Barber's work there were some exceptional cases which will be 

 mentioned later. He found many races of each, but each race was 

 constant, with the exceptions noted below. Long-continued selec- 

 tion had no effect in changing one of these races. Barber also studied 

 individuals having various peculiarities. While the vast majority 

 of these peculiarities behaved exactly as Jennings found them to 

 do in Parameciimi, he did find a few cases within a pure race (that 

 is, in the descendants of a single individual) that transmitted their 

 peculiarities to their descendants. Here we have actual evolu- 

 tionary change in a race. Races of yeast were produced having 

 cells of different form from the parent type and races of bacteria 

 composed of longer rods than the parents, but such cases were 

 extremely rare. Thus we must assume that there are occasionally 

 permanent evolutionary changes. As to the amount of change in such 

 cases we can get some information from Jennings's races of Para- 

 mecium, assmning, of course, that the differences between the various 

 races have come about by evolutionary change. The difference 

 between the average size of the two smallest races of Paramecium 

 studied by Jennings was only 0.00028 inch, yet the progeny of any 

 individual, large or small, in either of these two races, accurately 

 maintained this difference between the races. The important point 

 in all this is that when we are dealing with individuals of a pure race, 

 or, as Webber calls them, a ''clonal" race of variety — that is, indi- 

 viduals descended from a single individual b}^ vegetative propa- 

 gation — except for those very rare cases in which positive evolu- 

 tionary change occurs the fluctuating differences between indi- 

 viduals have absolutely no bearing on the evolutionary process. 

 According to Jennings there seems little doubt that this is true for 

 organisms in general. He says: 



In Paramecium, in the extensive study of many races for hundreds of generations 

 by exact statistical and experimental methods, not one single instance was observed 

 of variation in the sense of an actual change in the race. 



So far as the e^'idence goes every race is essentially the same throughout the work 

 and may have been the same for unnumbered ages. 



Jennings emphasizes the fact that real evolutionary changes do 

 not occur often or easily. ''The fundamental constitution of the 

 race is resistant to all sorts of influences. It changes only in excess- 

 ively rare instances and for unknown causes." 



In summarizing his conclusions, Jennings makes the following 

 statement: "Until some one can show that selection is effective 

 within pure lines it is only a statement of fact to say that all the 

 experimental evidence we have is against this." 



« Cited by Jennings in American Naturalist, June, 1909, 



165 



