560 THE AMEBIC AX XATURALIST [Vol. XL VII 



high, respectively. It is difficult to imagine fertile hybrids 

 of such parentage much more reduced in their vegetative 

 expression than are these dwarfs. Furthermore, the 

 reduction was apparently a complete loss in the power of 

 a greater growth, as was indicated by the dwarfs breed- 

 ing true in the F 3 generation. 



If the dwarfs were to be interpreted in so simple a 

 manner as recessives from a cross where two factors for 

 size were allelomorphic to their absence the ratio of the 

 dwarfs to the mass of the culture should have been as 

 1 : 15. Why then in the mass of the culture, dwarfs ex- 

 cluded, was there no evidence of other classes? The two 

 factors assumed must be of large value if their absence 

 is to make the difference between the size of the dwarfs, 

 3-4 dm., and the size of the parents, an average of about 

 15 dm. There might be expected a class of giants to bal- 

 ance the class of dwarfs and in the ratio of 1:15. 

 There should have been several other classes ranging 

 between these giants and the dwarfs. With only two 

 factors for size concerned, and these of such large value, 

 it seems impossible that the fluctuating variations could 

 conceal the presence of such classes. Yet the mass of 

 the culture failed to exhibit them, and only the dwarfs 

 could be separated as a class sufficiently distinct to war- 

 rant its designation. The mass of the culture ranged in 

 size approximately between the limits of the parents; 

 the gap between them and the dwarfs was not bridged 

 by intermediates. 



I am aware that the dwarfs might be explained as re- 

 sulting from the presence of an inhibiting factor intro- 

 duced into the cross, but again there should have been 

 evidence of other size classes together with the dwarfs 

 according as the inhibitor was present in a full or in a 

 lessened amount or was entirely absent. These difficul- 

 ties are in themselves of sufficient weight, let alone the 

 general improbability of such a situation. 



The explanation of the progressive evolution of an 

 F 2 generation in which the culture with respect to cer- 



