Recent Advances in the Study of Heredity. 259 
these yellows there are two types; one of which is the result of 
the union of two germ-cells, both bearing the yellow factor; the 
other of which is the result of the union of a germ-cell bearing the 
green factor with one bearing the yellow. The former type evidently 
corresponds to the (pure) yellow in F 2 which breeds true, the latter 
to the (hybrid) yellows whose offspring split up again into yellows 
and greens. And, inasmuch as, on this hypothesis, the greens 
present no difficulty, it will be seen that this theory is in perfect 
accord with observed results. 
Furthermore, the otherwise anomalous fact that the yellow 
hybrids continue to produce yellows and greens in the same 
proportions as we proceed away from F,, is explained; for, on this 
theory, the result of the union of two individuals is determined by 
the contents of the germ-cells borne by them ; and the gametic 
constitution of the hybrid in F 10 is identical with that of the F x 
hybrid. It is not necessary to point out in detail how all the other 
results which ensue from the union of germ-cells which differ in 
respect of a single pair of characters (in this instance, yellow and 
green) are in accord with Mendel’s theory; but it is worth while 
casting a glance at the basis on which the Mendelian prediction as 
to the result of the crucial experiment described on p. 177 rests. 
No difference between the result in F 2 of the union between a 
pure yellow and a pure green, and a union between a pure yellow 
and an extracted green in F 6 would be anticipated on the Mendelian 
hypothesis, because the contents of the germ-cells of a pure green 
and those of the germ-cells of an extracted green in F 5 are exactly 
the same, namely the green factor, and the green factor only. 
Before proceeding to a further consideration of this question 
of gametic constitution, let us pay attention to a feature of Mendel’s 
theory, which is so essential a part of it, that it is apt to he overlooked. 
It is that the theory is essentially a germinal theory of inheritance: 
it starts by assuming the existence of certain potentialities in the 
germ-cell and is in no way concerned with the question as to how 
they got there. It is a theory, therefore, which anyone who holds 
a Lamarckian theory of evolution must reject, unless he maintains 
that the conception of the relation between successive generations 
of organisms which that theory involves is not applicable to certain 
characters, of which cotyledon-colour in Pisum is one; or that the 
Mendelian experiments have not yet been continued for a number 
of generations sufficiently great to allow the machinery of inheritance, 
postulated by that theory, time to exert any measurable effect. This 
