636 
if at all nearer to the central mystery. We see nothing 
that we can analyse further—nothing that can be trans- 
lated into terms less inscrutable than the physiological 
events themselves. Not only does embryology give no 
direct aid, but the failure of cytology is, so far as I can 
judge, equally complete. The chromosomes of nearly 
related creatures may be utterly different both in 
number, size, and form. Only one piece of evidence 
encourages the old hope that a connection might be 
traceable between the visible characteristics of the body 
and those of the chromosomes. I refer, of course, to 
the accessory chromosome, which in many animals 
distinguishes the spermatozoon about to form a female 
in fertilisation. Even it, however, cannot be claimed 
as the cause of sexual differentiation, for it may be 
paired in forms closely allied to those in which it is 
unpaired or accessory. The distinction may be present 
or wanting, like any other secondary sexual character. 
Indeed, so long as no one can show consistent dis- 
tinctions between the cytological characters of somatic 
tissues in the same individual we can scarcely expect 
to perceive such distinctions between the chromosomes 
of the various types. 
For these methods of attack we now substitute 
another, less ambitious, perhaps, because less com- 
prehensive, but not less direct. If we cannot see 
how a fowl by its egg and its sperm gives rise to a, 
chicken or how a sweet pea from its ovule and its 
pollen grain produces another sweet pea, we at least 
can watch the system by which the differences between 
the various kinds of fowls or between the various 
kinds of sweet peas are distributed among the off- 
spring. By thus breaking the main problem up into 
its parts we give ourselves fresh chances. This 
analytical study we call Mendelian because Mendel 
was the first to apply it. To be sure, he did not 
approach the problem by any such line of reasoning 
as I have sketched. His object was to-determine the 
genetic definiteness of species; but though in his 
writings he makes no mention of inheritance, it is 
clear that he had the extension in view. By cross- 
breeding he combined the characters of varieties in 
mongrel individuals and set himself to see how these 
characters would be distributed among the individuals 
of subsequent generations. Until he began _ this 
analysis nothing but the vaguest answers to such a 
question had been attempted. The existence of any 
orderly system of descent was never even suspected. 
In their manifold complexity human characteristics 
seemed to follow no obvious system, and the fact 
was taken as a fair sample of the working of heredity. 
Misconception was especially brought in by de- 
scribing descent in terms of ‘‘blood.’? The common 
speech uses expressions such as consanguinity, pure- 
blooded, half-blood, and the like, which call up a 
misleading picture to the mind. Blood is in some 
respects a fluid, and thus it is supposed that this fluid 
can be both quantitatively and qualitatively diluted 
with other bloods, just as treacle can be diluted with 
water. Blood in primitive physiology being the 
peculiar vehicle of life, at once its essence and _ its 
corporeal abode, these ideas of dilution and com- 
pounding of characters in the commingling of bloods 
inevitably suggest that the ingredients of the mixture 
once combined are inseparable, that they can be 
brought together in any relative amounts, and in 
short that in heredity we are concerned mainly with 
a quantitative problem. Truer notions of genetic 
physiology are given by the Hebrew expression “ seed.”’ 
If we speak of a man as ‘‘of the blood-royal’’ we 
think at once of plebeian dilution, and we wonder 
how much of the royal fluid is likely to be ‘‘in his 
veins’’; but if we say he is ‘‘of the seed of Abraham ”’ 
we feel something of the permanence and indestructi- 
NO. 2338; Svouqoa|| 
NATURE 
| AUGUST 20, 1914 
bility of that germ which can be divided and scattered 
among all nations, but remains recognisable in type 
and characteristics after 4000 years. 
I knew a breeder who had a chest containing bottles 
of coloured liquids by which he used to illustrate the 
relationships of his dogs, pouring from one to another 
and titrating them quantitatively to illustrate their 
pedigrees. Galton was beset by the same kind of 
mistake when he promulgated his ‘* Law of Ancestral 
Heredity.”” With modern research all this has been 
cleared away. The allotment of characteristics among 
offspring is not accomplished by the exudation of 
drops of a tincture representing the sum of the char- 
acteristics of the parent organism, but by a process 
of cell-division, in which numbers of these characters, 
or rather the elements upon which they depend, are 
sorted out among the resulting germ-cells in an 
orderly fashion. What these elements, or factors as 
we call them, are we do not know. That they are in 
some way directly transmitted by the material of the 
ovum and of the spermatozoon is obvious, but it seems 
to me unlikely that they are in any simple or literal 
sense material particles. I suspect rather that their 
properties depend on some phenomenon of arrange- 
ment. However that may be, analytical breeding 
proves that it is according to the distribution of these 
genetic factors, to use a non-committal term, that the 
characters of the offspring are decided. The first 
business of experimental genetics is to determine their 
number and interactions, and then to make an 
analysis of the various types of life. 
Now the ordinary gcnealogical trees, such as those 
which the studbooks provide in the case of the 
domestic animals, or the Heralds’ College provides 
in the case of man, tell nothing of all this. Such 
methods of depicting descent cannot even show the 
one thing they are devised to show—purity of ** blood.”’ 
For at last we know the physiological meaning of 
that expression. An organism is pure-bred when it 
has been formed by the union in fertilisation of two 
germ-cells which are alike in the factors they bear; 
and since the factors for the several characteristics 
are independent of each other, this question of purity 
must be separately considered for each of them. A 
man, for example, may be pure-bred in respect of his 
musical ability and cross-bred in respect of the colour 
of his eyes or the shape of his mouth. Though we 
know nothing of the essential nature of these factors, 
we know a good deal of their powers. They may 
confer height, colour, shape, instincts, powers both 
of mind and body; indeed, so many of the attributes 
which animals and plants possess that We ice: justified 
in the expectation that with continued analysis they 
will be proved to be responsible for most if not all of 
the differences by which the varying individuals of 
any species are distinguished from each other. I will 
not assert that the greater differences which charac- 
terise distinct species are due generally to such inde- 
pendent factors, but that is the conclusion to which 
the available evidence points. All this is now so well 
understood, and has been so often demonstrated and 
expounded, that details of evidence are now super- 
fluous. 
But for the benefit of those who are unfamiliar with 
such work let me briefly epitomise its main features 
and consequences. Since genetic factors are definite 
things, either present in or absent from any germ- 
cell, the individual may be either ‘‘ pure-bred”’ for any 
particular factor, or its absence, if he is constituted 
by the union of two germ-cells both possessing or 
both destitute of that factor. If the individual is 
thus pure, all his germ-cells will in that respect be 
identical, for they are simply bits of the similar germ- 
cells which united in fertilisation to produce the parent 
