272 Heredity and Environment 



of the phenomena in question. It is possible that the divisions 

 of the cell body in these protozoans is not always into exactly 

 equivalent halves in which case variations might take place in 

 the descendants, which might then be heaped up by selection ; or 

 perhaps there are multiple or modifying factors in this case also, 

 so that selection has acted as in Castle's rats. 



Value of Selection. In conclusion, the evidence which is most 

 clear-cut and abundant indicates that selection by itself is unable 

 to change inheritance factors or to cause mutations. Nevertheless 

 selection is of great service in separating good lines or races from 

 poor ones, and this is the chief significance of the artificial selec- 

 tion practiced by breeders. 



The elimination of certain races by natural selection is an im- 

 portant factor in evolution though it has nothing to do directly with 

 the formation of new characters or new races but serves merely 

 as a sieve, as deVries has expressed it, to sort the individuals 

 which are supplied to it. Although selection has no power 

 to make or change characters, it preserves certain lines and elim- 

 inates others and thus fixes the type of a species. Finally the 

 elimination of the unfit by natural selection is still the only 

 mechanistic explanation of fitness, or adaptation, in organisms. . 



III. METHODS OF MODERN GENETICS 



I. Mendelian Association and Dissociation of Characters. 

 Breeders have long known that it is possible to get certain desir- 

 able characters of an organism from one race and others from 

 another race. But since the discovery of the Mendelian princi- 

 ples of heredity such new combinations of old characters have 

 been made repeatedly, and with almost the same certainty of 

 results as when the chemist makes combinations of elements or 

 compounds. 



In Fig. 95, A and B, are shown two guinea-pigs, one having 

 long (L), rough and tumbled (T), white (W) hair, and the other 

 having short (S), smooth (Sin), red (R) hair. When such ani- 

 mals are crossed one should get in the F 2 generation 27 genotypes 



