82 Life and Death, Heredity and Evolution 



sufficient thoroughness, so that there would be no real dif- 

 ference in this respect between them and Difflugia. I should 

 like to say, however, that from experience with Difflugia 

 and other organisms, and from the work of other investi- 

 gators, I am personally convinced that there is a difference 

 between organisms as to the frequency with which hereditary 

 variations occur. They occur on the whole relatively fre- 

 quently in Difflugia. In many other organisms the germinal 

 material is apparently so protected, and so precisely divided 

 at reproduction, that such changes are rare. 



And this brings us to the question as to just what in the 

 organism it is that is altered when the hereditary characters 

 change. A number of possibilities are open here. Hegner 1 

 (1919) discovered that in Arcella the hereditary size varies 

 with the number of nuclei or the amount of chromatin pres- 

 ent, and that these change at times as a result of irregulari- 

 ties in division. The number of spines further was found to 

 be related to the size; larger individuals have more spines 

 than smaller ones. Hence hereditary diversities in the num- 

 ber of spines also were brought about by alterations in the 

 number of nuclei or volume of chromatin. Some of the heredi- 

 tary changes in the characters of Difflugia may have been 

 brought about in the same way, but it is clear that most of 

 them were not. For as we have seen, some of the new lines 

 produced were small with large spines, some large with large 

 spines, the different characters being independent in their 

 hereditary diversities, so as to give stocks with different com- 

 binations of characters. These cannot be accounted for by 

 quantitative alterations in the amount of the nuclear material. 

 Another possibility lies in certain peculiarities of the nu- 

 cleus in such animals as Difflugia. There is in addition to one 

 or more very sharply defined nuclei a cloud-like mass of nu- 

 'Hegner, R. W., Proceedings of National Academy, January, 1919. 



