640 
further to confess that we cannot often actually prove 
variation by loss of factor to be a real phenomenon. 
Lotsy doubts whether even this phenomenon occurs. 
The sole source of variation, in his view, is crossing. 
But here I think he is on unsafe ground. When a 
well-established variety like ‘Crimson King” 
Primula, bred by Messrs. Sutton in’ thousands of in- 
dividuals, gives off, as it did a few years since, a 
salmon-coloured variety, ‘‘Coral King,’’ we might 
claim this as a genuine example of variation by loss. 
The new variety is a simple recessive. It differs from 
‘Crimson King’ only in one respect, the loss of a 
single colour-factor, and, of course, bred true from 
its origin. To account for the appearance of such a 
new form by any process of crossing is exceedingly 
difficult. From the nature of the case there can have 
been no cross since ‘‘Crimson King ’’ was established, 
and hence the salmon must have been concealed as a 
recessive from the first origin of that variety, even 
when it was represented by very few individuals, 
probably only by a single one. Surely, if any of these 
had been heterozygous for salmon this recessive could 
hardly have failed to appear during the process of 
self-fertilisation by which the stock would be multi- 
plied, even though that selfing may not have been 
strictly carried out. Examples like this seem to me 
practically conclusive.® They can be challenged, but 
not, I think, successfully. Then again in regard to 
those variations in number and division of parts which 
we call meristic, the reference of .these to original 
cross-breeding .is surely barred by the circumstances in 
which they often occur. There remain also the rare 
examples mentioned already in which a single wild 
origin may with much confidence be assumed. In 
spite of repeated trials, no one has yet succeeded in 
crossing the Sweet Pea with any other leguminous 
species. We know that early in its cultivated history 
it produced at least two marked varieties which I can 
only conceive of as spontaneously arising, though, 
no doubt, the profusion of forms we now have was 
made by the crossing of those original varieties. I 
mention the Sweet Pea thus prominently for another 
reason, that it introduces us to another though sub- 
sidiary form of variation, which may be described as 
a fractionation of factors. Some of my Mendelian 
colleagues have spoken of genetic factors as per- 
manent and indestructible. Relative permanence in 
a sense they have, for they commonly come out un- 
changed after segregation. But I am satisfied that 
they may occasionally undergo a quantitative dis- 
integration, with the consequence that varieties are 
produced intermediate between the integral varieties 
from which they were derived. These disintegrated 
conditions I have spoken of as subtraction—or reduc- 
tion—stages. For example, the Picotee Sweet Pea, 
with its purple edges, can surely be nothing but a 
condition produced by the factor which ordinarily 
makes the fully purple flower, quantitatively 
diminished. The pied animal, such as the Dutch 
rabbit, must similarly be regarded as the result of 
partial defect of the chromogen from which the pig- 
ment is formed, or conceivably of the factor which 
effects its oxidation. On such lines I think we may 
with great confidence interpret all those intergrading 
forms which breed true and are not produced by 
factorial interference. 
It is to be inferred that these fractional degradations 
are the consequence of irregularities in segregation. 
We constantly see irregularities in the ordinary 
meristic processes, and in the distribution of somatic 
differentiation... We are familiar with half segments, 
5 The numerous and most interesting ‘‘mutations’’ recorded by Prof. 
T. H. Morgan and his colleagues in the fly, Drosophila, may also be cited 
as unexceptionable cases. 
NO. 2338s, VOL. 193) 
NATURE 
[AUGUST 20, 1914 
| how 
with imperfect twinning, with leaves partially petaloid, 
with petals partially sepaloid. All these are evidences 
of departures from the normal regularity in the 
rhythms of repetition, or in those waves of differentia- 
tion by which the qualities are sorted out among the 
parts of the body. Similarly, when in segregation 
the qualities are sorted out among the germ-cells in 
certain critical cell-divisions, we cannot expect these 
differentiating divisions to be exempt from the im- 
perfections and irregularities which are found in all 
the grosser divisions that we can observe. If I am 
right, we shall find evidence of these irregularities 
in the association of unconformable numbers 
with the appearance of the novelties which I 
have called fractional.. In passing let us 
the history of the sweet pea belies those 
ideas of a continuous evolution with which we 
had formerly to contend. The big varieties came 
first. The little ones have arisen later, as I suggest 
by fractionation. Presented with a collection of 
modern sweet peas how prettily would the devotees 
of continuity have arranged them in a graduated 
series, showing how every intergrade could be found, 
passing from the full colour of the wild Sicilian 
species in one direction to white, in the other to the 
deep purple of ‘Black Prince,’ though happily we 
know these two to be among the earliest to have 
appeared. 
Having in and other considerations 
view these 
| which might be developed, I feel no reasonable doubt 
that though we may have to forgo a claim to varia- 
tions by addition of factors, yet variation both 
by loss of factors and by fractionation of factors 
is a genuine phenomenon of contemporary nature. 
If, then,. we have to dispense, as seems likely, 
‘with any addition from without we must begin 
seriously to consider whether the course of evolution 
can at all reasonably be represented as an unpacking 
of an original complex which contained within itself 
the whole range of diversity which living things 
present. I do not suggest that we should come to a 
judgment as to what is or is not probable in these 
respects. As I have said already, this is no time for 
devising theories of evolution, and I propound none. 
But as we have got to recognise that there has been 
an evolution, that somehow or other the forms of 
life have arisen from fewer forms, we may as well 
see whether we are limited to the old view that evolu- 
tionary progress is from the simple to the complex, 
and whether after all it is conceivable that the process 
was the other way about. When the facts of genetic 
discovery become familiarly known to _ biologists, 
and cease to be the preoccupation of a few, as they 
still are, many and long discussions must inevitably 
arise on the question, and I offer these remarks to 
prepare the ground. I ask you simply to open your 
minds to this possibility. It involves a certain effort. 
We have to reverse our habitual modes of thought. 
At first it may seem rank absurdity to suppose that 
the primordial form or forms of protoplasm could 
have contained complexity enough to produce the 
divers types of life. But it is easier to imagine that 
these powers could have been conveyed by extrinsic 
additions? Of what nature could these additions be? 
Additions of material cannot surely be in question. 
We are told that salts of iron in the soil may turn a 
pink hydrangea blue. The iron cannot be passed on 
to the next generation. How can the iron multiply 
itself? The power to assimilate the iron is all that 
can be transmitted. A disease-producing organism 
' like the pebrine of silkworms can in a very few cases 
be passed on through the germ-cells. Such an 
organism can multiply and can produce its char- 
acteristic effects in the next generation. But it does 
note 
— 
= re Ae 
