TRANSACTIONS OF SECTION D, 585 
it would be necessary to prove that he was exceptionally ignorant of breeding, 
though on the face of the evidence he seems sufficiently expert. 
A generalisation respecting the consequences of heterozygosis possessing greater 
value is this. When a pair of gametes unites in fertilisation the characters of the 
zygote depend directly on the constitution of these gametes, and not on that of 
the parents from which they came, To this generalisation we know as yet only 
two clear exceptions, These very curious cases are exactly alike in that, though 
segregation obviously occurs in a seed-character, the seeds borne by the hybrid (F,) 
all exhibit the hybrid character, and the consequences of segregation in the particular 
seed-character are not evident till the seeds (F,) of the second (F,) generation are 
determinable. Of these the first is the case of indent peas investigated especially by 
Tschermak, Crossed with wrinkled peas I have found the phenomena normal, but 
when the cross is made with a round type the exceptional phenomenon occurs. 
The second case is that discovered by Biffen in the cross between the long-grained 
wheat called Polish and short-grained Rivett wheat, demonstrations of which will 
be laid before you. No satisfactory account of these peculiarities has been yet 
suggested, but it is evident that in some unexplained way the maternal plant- 
characters control the seed-characters for each generation. It is, of course, likely 
that other comparable cases will be found. 
Appearances have been seen in at least four cases (rats, mice, stocks, sweet peas) 
suggesting at first sight that a heterozygosis between two gametes, both extracted, 
may give, e.g., dominance ; while if one, or both, were pure, they would give a rever- 
sionary heterozygote. If this occurrence is authenticated on a sufficient scale, we 
shall of course recognise that the fact proves the presence in these cases of some 
pervading and non-segregating quality, distributed among the extracted gametes 
formed by the parent heterozygote. As yet, however, I do not think the evidence 
enough to warrant the conclusion that such a pervading quality is really present, 
and I incline to attribute the appearances to redistribution of characters belonging 
to independent pairs in the manner elucidated by Cuénot. The point will be 
easily determined, and meanwhile we must note the two possibilities. 
Following, therefore, our first proposition that the gametes belong to definite 
classes, comes the second proposition, that the unions of members of the various 
classes have specific consequences. Nor is this proposition simply the truistical 
statement that different causes have different effects; for by its aid we are led at 
once to the place where the different cause is to be sought—Gametogenesis. 
While formerly we hoped to determine the offspring by examining the ancestry of 
the parents, we now proceed by investigating the gametic composition of the 
parents. Individuals may have identical ancestry (and sometimes, to all appear- 
ances, identical characters), but yet be quite different in gametic composition; and, 
conversely, individuals may be identical in gametic composition and have very 
different ancestry. Nevertheless, those that are identical in gametic composition 
are the same, whatever their ancestry, Therefore, where such cases are concerned, 
in any considerations of the physiology of heredity, ancestry is misleading and 
passes out of account. To take the crudest illustration: if a hybrid is made 
between two races, A, B, and another hybrid between two other races, OC, D, it 
might be thought that when the two hybrids AB and CD are bred together, four 
races, A, B, C, and D, will be united in their offspring. This expectation may be 
entirely falsified, for the cell-divisions of gametogenesis may have split A from B 
and C from D, so that the final product may contain characters of only two races 
after all, being either AC, BO, AD, or BD. In practice, however, we are generally 
dealing with groups of characters, and the union of all the A group, for instaince, 
with all the C group will be a rare coincidence. 
It is the object of Mendelian analysis to state each case of heredity in terms 
of gametic composition, and thence to determine the laws governing the distri- 
bution of characters in the cell-divisions of gametogenesis. 
There are, of course, many cases which still bafile our attempts at such analysis, 
but some of the most paradoxical exceptions have been reduced to order by the 
accumulation of facts, The consequences of heterozygosis are curiously specific, 
