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THE AMERICAN NATURALIST 



[Vol. XLIX 



on the elimination suggested by "all existing progeny." 

 The extraordinary diminution in length (140 to 123.71 

 /a.) suggests some disturbing factor of metabolism. 



The results of the experiment with a "wild culture" 

 where progeny of "unpaired" and "paired" forms of P. 

 caudatum (?) were considered, again suggested to Jen- 

 nings the greater size of the progeny of the paired indi- 

 viduals (conjugants), a condition which was particularly 

 evident in the first generation. But it must be noted that 

 the disturbance of the function of conjugation in "un- 

 pairing" may have produced the result. The progeny of 

 the "unpairs" were relatively becoming larger from the 

 first to the seventh generation. These facts taken to- 

 gether with the absence of measurements of mean diam- 

 eters by which to calculate the mean volumes, suggest 

 that such a conclusion based on that part of the work 

 could not be accepted, and that the data strongly support 

 the proposition directly contrary to Jennings that the 

 progeny of conjugants tend to become smaller than the 

 progeny of non-conjugants although the latter may be 

 larger directly after conjugation as a result of slower 

 fission. Thus the evidence from various sources, 

 although incomplete, suggests that cross-bred unicellular 

 organisms are smaller than close-bred forms. 



Among multicellular organisms however it has long 

 been recognized that hybrids usually grew to a larger 

 size than either parental form, as has been observed by 

 Kohlreuter ('63), Knight ('99), Gartner ('49), as well 

 as Darwin, Mendel and others, although the cause of the 

 increased growth has been purely conjectural. It is quite 

 evident that the result is due to either the increased num- 

 ber of cells, a suggestion made by East, to the increased 

 size of the cells, or to the combination of both conditions. 



The question immediately arises as to the cause of the 

 increased size and vigor among cross-bred multicellular 

 organisms when the evidence indicates that cross-bred 

 unicellular organisms are smaller instead of larger. 



