AUGUST 20, 1914| 
organism. We thus reach the essential principle, that 
an organism cannot pass on to offspring a factor which 
it did not itself receive in fertilisation. Parents, 
therefore, which are both destitute of a given factor 
can only produce offspring equally destitute of it; 
and, on the contrary, parents both pure-bred for the 
presence of a factor produce offspring equally pure- 
bred for its presence. Whereas the germ-cells of the 
pure-bred are all alike, those of the cross-bred, which 
results from the union of dissimilar germ-cells, are 
mixed in character. Each positive factor segregates 
from its negative opposite, so that some germ-cells 
carry the factor and some do not. Once the factors 
have been identified by their effects, the average com- 
position of the several kinds of families formed from 
the various matings can be predicted. 
Only those who have themselves witnessed the 
fixed operations of these simple rules can feel their 
full significance. We come to look behind the simu- 
lacrum of the individual body, and we endeavour to 
disintegrate its features into the genetic elements by 
whose union the body was formed. Set out in cold 
general phrases, such discoveries may seem remote 
from ordinary life. Become familiar with them, and 
you will find your outlook on the world has changed. 
Watch the effects of segregation among the living 
things with which you have to do—plants, fowls, dogs, 
horses, that mixed concourse of humanity we call the 
English race, your friends’ children, your own chil- 
dren, yourself—and however firmly imagination be 
restrained to the bounds of the known and the proved, 
you will feel something of that range of insight into 
nature which Mendelism has begun to give. The 
question is often asked whether there are not also 
in operation systems of descent quite other than those 
contemplated by the Mendelian rules. I myself have 
expected such discoveries, but hitherto none have been 
plainly demonstrated. It is true we are often puzzled 
by the failure of a parental type to reappear in its 
completeness after a cross—the merino sheep or the 
fantail pigeon, for example. These exceptions may 
still be plausibly ascribed to the interference of a 
multitude of factors, a suggestion not easy to dis- 
prove; though it seems to me equally likely that 
segregation has been in reality imperfect. Of the 
descent of quantitative characters we still know prac- 
tically nothing. These and hosts of difficult cases 
remain almost untouched. In particular the discovery 
of E. Baur, and the evidence of Winkler in regard to 
his ‘graft hybrids,’? both showing that the sub- 
epidermal layer of a plant—the layer from which the 
germ-cells are derived—may bear exclusively the 
characters of a part only of the soma, give hints of 
curious complications, and suggest that in plants at 
least the interrelations between soma and gamete may 
be far less simple than we have supposed. Never- 
theless, speaking generally, we see nothing to indi- 
cate that qualitative characters descend, whether in 
plants or animals, according to systems which are 
incapable of factorial representation. 
The body of evidence accumulated by this method 
of analysis is now very large, and is still growing fast 
by the labours of many workers. Progress is also 
beginning along many novel and curious lines. The 
details are too technical for inclusion here. Suffice it 
to say that not only have we proof that segregation 
affects a vast range of characteristics, but in the 
course of our analysis phenomena of most unexpected 
kinds have been encountered. Some of these things 
twenty years ago must have seemed inconceivable. 
For example, the two sets of sex organs, male and 
female, of the same plant may not be carrying the 
same characteristics; in some animals characteristics, 
NO. 2338, VOL. 93| 
NATURE 
| polarity shown 
637 
quite independent of sex, may be distributed solely 
or predominantly to one sex; in certain species the 
male may be breeding true to its own type, while 
the female is permanently mongrel, throwing off eggs 
of a distinct variety in addition to those of its own 
type; characteristics, essentially independent, may be 
associated in special combinations which are largely 
retained in the next generation, so that among the 
grandchildren there is numerical preponderance of 
those combinations which existed in the grandparents 
—a discovery which introduces us to a new pheno- 
menon of polarity in the organism. 
We are accustomed to the fact that the fertilised 
egg has a polarity, a front and hind end, for example; 
but we have now to recognise that it, or the primitive 
germinal cells formed from it, may have another 
in the groupings of the parental 
elements. I am entirely sceptical as to the occurrence 
of segregation solely in the maturation of the germ- 
cells, preferring at present to regard it as a special 
case of that patch-work condition we see in so many 
plants. These mosaics may break up, emitting bud- 
sports at various cell-divisions, and I suspect that the 
great regularity seen in the F, ratios of the cereals, 
for example, is a consequence of very late segregation, 
whereas the excessive irregularity found in other cases 
may be taken to indicate that segregation can happen 
at earlier stages of differentiation. 
The paradoxical descent of colour-blindness and 
other sex-limited conditions—formerly regarded as an 
inscrutable caprice of nature—has been represented 
with approximate correctness, and we already know 
something as to the way, or, perhaps, I should say 
ways, in which the determination of sex is accom- 
plished in some of the forms of life—though, I hasten 
to add, we have no inkling as to any method by which 
that determination may be influenced or directed. It 
is obvious that such discoveries have bearings on most 
of the problems, whether theoretical or practical, in 
which animals and plants are concerned. Permanence 
or change of type, perfection of type, purity or mixture 
of race, ‘racial development,” the succession of forms, 
from being vague phrases expressing matters of 
degree, are now seen to be capable of acquiring physio- 
logical meanings, already to some extent assigned 
with precision. For the naturalist—and it is to him 
that I am especially addressing myself to-day—these 
things are chiefly significant as relating to the history 
of organic beings—the theory of evolution, to use our 
modern name. They have, as I shall endeavour to 
show in my second address to be given in Sydney, an 
immediate reference to the conduct of human society. 
I suppose that everyone is familiar in outline with 
the theory of the origin of species which Darwin 
promulgated. Through the last fifty years this theme 
of the Natural Selection of favoured races has been 
developed and expounded in writings innumerable. 
Favoured races certainly can replace others. The 
argument is sound, but we are doubtful of its value. 
For us that debate stands adjourned. We go to 
Darwin for his incomparable collection of facts. We 
would fain emulate his scholarship, his width and his 
power of exposition, but to us he speaks no more 
with philosophical authority. We read his scheme of 
evolution as we would those of Lucretius or of 
Lamarck, delighting in their simplicity and_ their 
courage. The practical and experimental study of 
variation and heredity has not merely opened a new 
field; it has given a new point of view and new 
standards of criticism. Naturalists may still be found 
3 The fact that in certain plants the male end female organs respectively 
carry distinct factors may be quoted as almost decisively negativing the 
suggestion that segregation is confined to the reduction division. 
