182 PROCEEDINGS OF THE ACADEMY OF [May, 



which were made with the camera lucida, were index ±.10, ratio- 

 ±.05. These were so much larger in the case of the small shells 

 because of the difficulty in orienting them in exactly the same plane 

 at the time of measurement. 



The range of the offspring are almost the same as that of the parent 

 colony. This is only significant as showing what we may expect 

 when we are able to deal with larger numbers in a more precise 

 manner. 



Baker (1911) observed copulation between different individuals of 

 Lymncea stagnalis, Lymncea emarginata and Lymncea lanceata. Several 

 times L. emarginata tried to copulate with L. lanceata, & much smaller 

 animal. Baker saw also L. stagnalis in copulation with emarginata . 

 Heynemann (1869) records a cross between auricularia and peregra, 

 the former acting as female and the latter as male. Chaster (1909) 

 records a cross between L. stagnalis and L. auricularia, the former 

 taking the part of the male. The progeny were good examples of 

 peregra, and the query is raised by Chaster as to whether peregra 

 may not be the ancestral form of auricularia and stagnalis. Be that 

 as it may, it is possible that many of the so-called species and varieti' - 

 of Lymncea may be hybrids, and it will be necessary to experiment 

 in order to determine how true this is. In the ponds about Phila- 

 delphia, however, where Lymncea columella is found, the writer has 

 never found any other species present. The chance of hybridization 



is, therefore, rare. 



Conclusion. 



Johannsen (1911) clearly defined the "pure line." "A pure line.'' 

 he said, "may^be defined as the descendants from one single 

 homozygotic organism exclusively propagating by self-fertiliza- 

 tion. ... A line ceases to be 'pure' when hybridization (or 

 even intercrossing) disturbs the continuity of the self-fertilization." 

 With this definition in mind it is necessary, if we would have a pure 

 line in the Johannsenian sense, to deal with hermaphroditic organisms. 

 Not only must the organisms be hermaphroditic, but self-fertilization 

 must be possible. It must either be normal or can be induced and 

 controlled. These conditions can easily be fulfilled in many plants, 

 but, as far as I am aware, no animals with the proper requirements 

 have been experimented upon. 



To be sure, Jennings (1911) and others have traced "lines" of 

 Protozoa. These animals divide by fission so that their method of 

 reproduction is probably closely allied to asexual or to partheno- 

 genetic reproduction as found in multicellular forms. 



The Hydra with which Handel (1907) worked reproduced by 



