232 
Allen. — Nuclear Division in the 
The rule as to the similarity among individuals of the same cross is by 
no means of so general application as that of the constancy of individual 
characters during ontogeny. Nageli (quoted by Swingle and Webber, ’98) 
finds that * in general the hybrids in the first generation vary the less the 
more distantly related the parent forms are ; that is, the specific hybrids 
vary less than the varietal hybrids, the former often being characterized by 
great uniformity, the latter by great diversity of form.’ And Swingle and 
Webber note the general fact that £ races of cultivated plants, even though 
very diverse, produce very variable hybrids in the first generation, while 
usually by crossing wild species closely resembling each other hybrids are 
obtained which are constant in the first generation.’ Similarity between 
hybrids of the same parentage is illustrated by most of the cases already 
considered, notably by Mendel’s Pisam hybrids. Instances of variation 
are furnished by the Hieracium hybrids produced by Mendel (’69) ; also 
(Swingle and Webber, ’98) by the two different crosses obtained by 
Gartner between Nicotiana quadrivalvus and N. macrophylla. 
Satisfactory evidence upon this point is, however, difficult to obtain, 
for the reason that the parents of any particular individual were themselves 
sexually produced ; each of them received from its parents two different 
idioplasms, and the generation with which we are directly concerned is 
affected by the unequal distribution among its members of these grand- 
parental idioplasms. In determining the amount of variation among the 
individuals produced by a given cross it is, therefore, necessary to be certain 
that the parental idioplasms were pure as respects the particular qualities 
under consideration. Absolute certainty upon this point is probably always 
impossible, and even approximate certainty is out of the question unless, as 
is usually not the case, we know the pedigree of the parents for many genera- 
tions. In cases of variation among hybrids of the same cross, therefore, 
it is necessary always to take into consideration the possibility that the 
variation is due to ancestral influences, a possibility which is especially 
strong in the case of cultivated races, and which, no doubt, accounts for 
much of the variation noted among hybrids of such races. It is very 
possible that, if perfect certainty with respect to ancestry were attainable, 
uniformity among offspring of the same cross between pure-bred parents 
would be found much more nearly universal than at present it appears 
to be. 
A special case, to be classed with those just discussed, is afforded by 
the fact that, in most cases, it makes no difference in which direction the 
cross takes place — that is, a cross between A male and B female gives 
the same results as one between A female and B male. This seems to 
imply that differences in the source of the respective idioplasms do not, by 
differences in their relative position within the nucleus or otherwise, affect 
their capacity for determining or influencing external characters. Occasional 
