486 



SCIENCE 



[N. S. Vol. XXXIX. No. 1005 



iological limit, even though the results 

 hold for a single generation, have the great- 

 est practical value for the future of agri- 

 culture and animal breeding. 



It should first be noted that size in a 

 unicellular organism is dependent on the 

 absolute size of the individual cell with a 

 limit undoubtedly imposed by laws govern- 

 ing the ratio between volume and surface 

 in connection with osmosis. In multicellu- 

 lar organisms, however, size characters may 

 depend upon either the size or the number 

 of the component cells or upon both factors. 

 This distinction possibly explains an ap- 

 parent diversity in results obtained in the 

 two groups. 



Darwin, Mendel and others who have 

 seriously considered the question have rec- 

 ognized that hybrids, among plant forms 

 in particular, usually grew to a larger size 

 than either parental form, a result prob- 

 ably due to the increased rapidity of 

 cell division and consequently greater num- 

 ber 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 

 volume were concerned. Jennings (1911) 

 in his study of Paramecium reached a con- 

 trary 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 {V ^=l/'o-7tld^) by which 

 it may be demonstrated that the non-con- 

 jugant forms, while smaller than the others 

 at the beginning of the experiment, actu- 

 ally became larger. Thus in agreement 

 with the zygospores of Spirogyra, con- 

 jugation decreased 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 

 unicellular forms are smaller instead of 

 larger. Some investigations that I have 

 undertaken indicate an answer apparently 

 meeting the conditions. "While sufficient 

 control experiments have not been made to 

 venture more than a provisional opinion, 

 the data suggest that the cells of cross-bred 

 multicellular organisms are actually 

 smaller than the cells of inbred or pure line 

 forms, and that the more rapid division is 

 a function of the greater ratio surface has 

 to volume in a small cell with the better 

 opportunity thus obtained for increased 

 metabolism. 



That there is need of further investiga- 

 tion on size and variability in pure lines 

 and in cross-bred forms through the appli- 

 cation of statistical methods in connection 

 with the maintenance of pedigrees through 

 long series of generations seems evident. 

 Eventually theories will make way for facts 

 which will allow a proper perspective. 



Where do the results presented in the 

 preceding pages lead us 1 Does their value, 

 so far as their bearing upon the production 

 of new and transmissible characters that 

 will build up an organism in a required 

 direction, consist merely in the formulating 

 of hypothesis after hypothesis which as in- 

 vestigations proceed will in turn make way 

 for other hypotheses equally transient ? Or, 

 on the other hand, do they mark a definite 

 progress along the lines we are endeavoring 

 to follow, namely, the control of evolution. 



Before attempting a reply which must 

 prove more or less unsatisfactory to those 

 looking forward to immediate results, it 

 seems advisable to pause for a moment and 

 in the light of the preceding discussion con- 



