PRESIDENTIAL ADDRESS. 393 
in its arrangement in more or less multiform chromosomes during mitosis. We 
may provisionally accept Weismann’s view that these chromosomes are them- 
selves heterogeneous, being composed of chromomeres or ids, which in their 
turn are composed of determinants. 
All this complexity of structure may be attributed to the effects of oft- 
repeated amphimixis, a view which is supported in the most striking manner by 
the fact that the nucleus in all ordinary somatic cells (in animals and in the 
diploid generation of plants) has a double set of chromosomes, one derived from 
the male and the other from the female parent, and by the well-known 
phenomenon of chromatin reduction which always precedes amphimixis. 
When we approach the problem of heredity from the experimental side we 
get very strong evidence of the existence in the germ-plasm of definite material 
substances associated with the inheritance of special characters. | Mendelian 
workers generally speak of these substances as factors, but the conception of 
factors is evidently closely akin to that of Weismann’s hypothetical determinants, 
The cytological evidence fits in very well with the view that the factors in ques- 
tion may be definite material particles, and it is quite possible that such particles 
may have a specific chemical constitution to which their effects upon the 
developing organism are due. 
From our point of view the interesting thing is the possibility that arises 
through the sexual process of the permutation and combination of different 
factors derived from different lines of descent. A germ-cell may receive addi- 
tions to its collection of factors or be subject to subtractions therefrom, and in 
either case the resulting organism may be more or less conspicuously modified. 
By applying the method of experimental hybridisation a most fruitful and 
apparently inexhaustible field of research has been opened up in this direction, 
in the development of which no one has taken a more active part than the present 
President of the British Association. There cannot be the slightest doubt. that 
a vast number of characters are inherited in what is called the Mendelian 
manner, and, as they are capable of being separately inherited and interchanged 
with others by hybridisation, we are justified in believing that they are separately 
represented in the germ-cells by special factors. Important as this result is, 
I believe that at the present time there exists a distinct danger of exaggerating 
its significance. The fact that many new and apparently permanent combinations 
of characters may arise through hybridisation, and that the organisms thus pro- 
duced have all the attributes of what we call distinct species, does not justify us 
in accepting the grotesque view—as it appears to me—that all species have arisen 
by crossing, or even the view that the organism is entirely built up of separately 
transmissible ‘unit characters.’ 
Bateson tells us that ‘Baur has for example crossed species so unlike as 
Antirrhinum majus and molle, forms differing from each other in almost every 
feature of organisation.’ Surely the latter part of this statement cannot be cor- 
rect, for after all Antirrhinum majus and molle are both snapdragons, and 
exhibit all the essential characters of snapdragons. 
T think it is a most significant fact that the only characters which appear 
to be inherited in Mendelian fashion are comparatively trivial features of the 
organism which must have arisen during the last stages of phylogeny. This is 
necessarily the case, for any two organisms sufficiently nearly related to be 
capable of crossing are identical as regards the vast majority of their characters. 
It is only those few points in which they differ that remain to be experimented 
on. Moreover, the characters in question appear to be all non-adaptive, having 
no obvious relation to the environment and no particular value in the struggle 
for existence. They are clearly what Weismann calls blastogenic characters, 
originating in the germ-plasm, and are probably identical with the mutations 
of de Vries. These latter are apparently chromatin-determined characters, for, 
as Dr, Gates has recently shown in the case of @nothera, mutation may result 
from abnormal distribution of the chromosomes in the reduction division.° 
We have next to inquire whether or not the Mendelian results are really in 
any way inconsistent with the general theory of evolution outlined in the earlier 
part of this address. Here we are obviously face to face with the old dispute 
between epigenesis and preformation. The theory of ontogeny which I first: 
° Quarterly Journal of Microscopical Science, vol. 59, p. 557. 
