97 
1888-89.] Mr J. A. Thomson on Theory of Heredity. 
ovum which is destined to give rise in the offspring to the 
cell which corresponds to the one which produced the gem- 
mule, or else it unites with a closely-related particle, destined 
to give rise to a closely-related cell. Such a cell will he a 
hybrid, tending to vary. 
(7) As the ovarian ova of the offspring share by direct inheritance 
all the properties of the fertilised ovum, the organisms to 
which they give rise will tend to vary in the same way. 
(8) A cell which has thus varied will continue to throw off 
gemmules, and thus to transmit variability to the corre- 
sponding part in the bodies of successive generations of 
descendants until a favourable variation is seized upon by 
natural selection. 
(9) As the ovum which produced this selected organism will 
transmit the same variation to its ovarian ova by direct 
inheritance, the characteristic will be established as specific, 
and transmitted henceforth without gemmules. 
The above theory, being important, has been stated at some length. 
Apart from the suggestion of variation as due to sexual intermingling, 
with which j.Weismann has made us more familiar ; apart, too, from 
the suggestion of germinal continuity, the credit of which Brooks 
shares, there are several important points to be emphasised in the 
modification proposed. It is in unwonted and abnormal condi- 
tions that the cells of the body throw off gemmules. The male 
elements are the special centres of their accumulation ; the female 
it is that keeps up the general resemblance between offspring and 
parent. 
It is not proposed to enter into criticism of pangenesis theories. 
The best criticism is found in that abandonment of special hypo- 
theses which recent advances have rendered possible. It has often 
been urged that the hypothesis of pangenes is involves not one but 
many suppositions — that it is just as difficult to understand why a 
gemmule should reproduce a cell like its own origin as to under- 
stand the entire problem, and so on. Detailed criticism will be 
found in the works of Galton, Ribot, Brooks, Herdman, Plarre, 
and others. It is enough for us to emphasise the comparative 
gratuitousness of any special theory whatever, a paradox which is 
explained in the succeeding section. 
VOL. XVI, 25/3/89 
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