PRESIDENT’S ADDRESS. 2D 
instances of imperfect segregation. The series of germ-cells produced 
by the cross-bred consists of some with no black, some with full black, 
and others with intermediate quantities of black. No statistical tests 
of the condition of the gametes in such cases exist, and it is likely that 
by choosing suitable crosses all sorts of conditions may be found, 
ranging from the simplest case of total segregation, in which there are 
only two forms of gametes, up to those in which there are all inter- 
mediates in various proportions. This at least is what general experi- 
ence of hybrid products leads me to anticipate. Segregation is 
somehow effected by the rhythms of cell-division, if such an expression 
may be permitted. In some cases the whole factor is so easily separated 
that it is swept out at once; in others it is so intermixed that gametes of 
all degrees of purity may result. That is admittedly a crude metaphor, 
but as yet we cannot substitute a better. Be all this as it may, there are 
many signs that in human heredity phenomena of this kind are common, 
whether they indicate a multiplicity of cumulative factors or imper- 
fections in segregation. Such phenomena, however, in no way detract 
from the essential truths that segregation occurs, and that the organism 
cannot pass on a factor which it has not itself received. 
In human heredity we have found some examples, and I believe 
that we shall find many more, in which the descent of factors is limited 
by sex. The classical instances are those of colour-blindness and 
hemophilia. Both these conditions occur with much greater frequency 
in males than in females. Of colour-blindness at least we know that 
the sons of the colour-blind man do not inherit it (unless the mother 
is a transmitter) and do not transmit it to their children of either 
sex. Some, probably all, of the daughters of the colour-blind father 
inherit the character, and though not themselves colour-blind, they 
transmit it to some (probably, on an average, half) of their offspring 
of both sexes. For since these normal-sighted women have only 
received the colour-blindness from one side of their parentage, only 
half their offspring, on an average, can inherit it. The sons who 
inherit the colour-blindness will be colour-blind, and the inheriting 
daughters become themselves again transmitters. Males with 
normal colour-vision, whatever their own parentage, do not have colour- 
blind descendants, unless they marry transmitting women. There 
are points still doubtful in the interpretation, but the critical fact is 
clear, that the germ-cells of the colour-blind man are of two kinds: 
(i) those which do not carry on the affection and are destined to take 
part in the formation of sons; and (ii) those which do carry on the 
colour-blindness and are destined to form daughters. There is evidence 
that the ova also are similarly predestined to form one or other of the 
sexes, but to discuss the whole question of sex-determination is beyond 
my present scope. The descent of these sex-limited affections never- 
