INHERITANCE 729 



racial mean in the same direction as do the selected parents, but not so 

 far. Only a small part of the selected parents' divergence from the mean 

 is inherited by the offspring. 



To what is due such inheritance, with regression toward the racial 

 mean? The interpretation is not clear, but the most natural conception 

 appears to be the following. The divergence from the racial mean in the 

 case of the selected parents is partly a matter of environment, partly a 

 matter of genetic constitution. The latter is inherited by offspring, the 

 former not. The fact that a part of the parent's peculiarity is inherited 

 shows that variation in the genetic constitution itself occurs in vegetative 

 reproduction and is inherited. It is, however, in large degree masked by 

 variations due to environmental conditions, and not inherited. 



As a consequence of the inheritance of some portion of the parents' 

 peculiarities, in time the single clone or biotype may by selective breeding 

 become differentiated into a number of biotypes, differing slightly in 

 inherited characters. Five such hereditarily different biotypes of Diffiugia 

 corona, produced through selective breeding from a single one, are 

 described and figured in the paper of Jennings (1916). 



For such heritable change in the genetic constitution two types of 

 interpretation may be suggested. On the one hand, the heritable changes 

 may be due to obscure alterations in environmental conditions, so that 

 these changes are comparable to those in acclimatization or other Dauer- 

 modifkatwnen. On the other hand, the changes may be attributed to 

 irregularities occurring in the genetic materials — either "mutations" or 

 slight irregularities of distribution — when reproduction occurs. At pres- 

 ent it appears not possible to decide between these two interpretations. 



SUMMARY AND INTERPRETATION 



To recapitulate, in Protozoa, a number of different types of inherited 

 changes may occur during vegetative reproduction, giving rise to indi- 

 viduals or lines of descent that have different characteristics which are 

 transmitted to descendants in further vegetative reproduction. 



First, there are changes that occur in the course of the normal life 

 history — changes as to sexual maturity, and, in some species, profound 

 alterations in structural and physiological characters ("alternation of 

 generations"). Each stage in such a life history includes many successive 



