114 THE SHORTER SCIENTIFIC PAPERS 



pared with the more normal rate of fission among non- 

 conjugants, this could result in nothing but an increased 

 variability, having, however, no bearing on the question 

 at issue. 



At the present time, therefore, it would seem that the 

 preponderance of evidence demonstrates that variability- 

 is decreased in cross breeding. 



2. Comparative Size 



The zygospores produced by close breeding have a 

 mean length of 62.38 /x. ± .18 ft. with a mean diameter 

 of 29.66fx. ± .lOjLi. and those produced by cross breeding 

 have a mean length of 60.44 /x. ± .13 fx. with a mean 

 diameter of 29.725 fx. ± .08 fi. Thus so far as length is 

 concerned the close bred zygospores are relatively 3.2 

 per cent larger and although slightly smaller in diameter, 

 when volume is considered by utilizing the formula 

 (V = y^nld^) the close bred forms are also 2.8 per cent 

 larger. Since these results are not in accord with the 

 general belief that cross fertilization increases size and 

 vigor, terms which have a diverse usage, however, it will 

 be well to consider other evidence bearing on the problem 

 with a view of attempting an explanation which may 

 meet the conditions imposed. 



Pearl ('07) in studying the conjugation of Paramecitifn 

 with particular reference to assortative mating, notes 

 that "the conjugant individuals when compared with the 

 non-conjugant, are shorter and narrower" and stated in 

 accordance with Calkins ('02) that the reduction in size 

 was quite probably dependent on functional changes 

 connected with reproduction. In Spirogyra, however, 

 both the close-bred and the cross-bred zygospores go 

 through similar reproductive processes in consequence of 

 which one may question the theory that the method of 

 conjugation is the decisive factor in bringing about the 

 result even in Paramecium. 



Jennings ('11) in comparing the size of conjugant and 

 non-conjugant Paramecium^ stated that 



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

 of non-conjugants and the diflference appears to be significant. 



