LEE BARKER WALTON 89 



of cross breeding or conjugation is of decided interest in- 

 asmuch as facts bearing on the solution of the problem as 

 to how size may be increased to the physiological limit, 

 even though the results hold for a single generation, have 

 the greatest practical value for the future of agriculture 

 and animal breeding. 



It should first be noted that size in a unicellular organ- 

 ism is dependent on the absolute size of the individual cell 

 with a limit undoubtedly imposed by laws governing the 

 ratio between volume and surface in connection with 

 osmosis. In multicellular organisms, however, size char- 

 acters may depend upon either the size or the number of 

 the component cells or upon both factors. This distinc- 

 tion possibly explains an apparent diversity in results ob- 

 tained in the two groups. 



Darwin, Mendel and others who have seriously con- 

 sidered the question have recognized that hybrids, among 

 plant forms in particular, usually grew to a larger size 

 than either parental form, a result probably due to the 

 increased rapidity of cell division and consequently 

 greater number of cells as conjectured by East. In the 

 study of zygospores of Spirogyra it was therefore noticed 

 with some interest that the cross-bred forms were smaller 

 than the close-bred forms so far as both length and vol- 

 ume were concerned. Jennings (1911) in his study of 

 Paramecmi7t reached a contrary conclusion, stating that 

 "The progeny of conjugants . . . were a little larger than 

 the progeny of non-conjugants and the difference appears 

 to be significant." This is correct merely in reference to 

 length, however, and that it is not true for actual size as 

 indicated by volume is evident on applying the formula 

 for the volume of a prolate spheroid {¥="!/ 6TTld'^) by 

 which it may be demonstrated that the non-conjugant 

 forms, while smaller than the others at the beginning of 

 the experiment, actually became larger. Thus in agree- 

 ment with the zygospores of Spirogyra, conjugation de- 

 creased size. 



The question immediately occurs as to the cause of the 

 increased size and vigor among cross-bred multicellular 

 organisms when the evidence indicates that cross-bred 



