638 
expounding teleological systems* which would have 
delighted Dr. Pangloss himself, but at the present time 
few are misled. The student of genetics knows that 
the time for the development of theory is not yet. 
He would rather stick to the seed-pan and the incu- 
Bator. 
In face of what we now know of the distribution of 
variability in nature, the scope claimed for natural 
selection in determining the fixity of species must be 
greatly reduced. The doctrine of the survival of the 
fittest is undeniable so long as it is applied to the 
organism as a whole, but to attempt by this principle 
to find value in all definiteness of parts and functions, 
and in the name of science to see fitness everywhere, 
is mere eighteenth-century optimism. Yet it was in 
application to the parts, to the details of specific differ- 
ence, to the spots on the peacock’s tail, to the colouring 
of an orchid flower, and hosts of such examples, that 
the potency of natural selection was urged with the 
strongest emphasis. Shorn of these pretensions the 
doctrine of the survival of favoured races is a truism, 
helping scarcely at all to account for the diversity of 
species. Tolerance plays almost as considerable a part. 
By these admissions almost the last shred of that 
teleological fustian with which Victorian philosophy 
loved to clothe the theory of evolution is destroyed. 
Those who would proclaim that whatever is is right 
will be wise henceforth to base this faith frankly on 
the impregnable rock of superstition and to abstain 
from direct appeals to natural fact. 
My predecessor said last year that in physics the 
age is one of rapid progress and profound scepticism. 
In at least as high a degree this is true of biology, 
and as a chief characteristic of modern evolutionary 
thought we must confess also to a deep but irksome 
humility in presence of great vital problems. Every 
theory of evolution must be such as to accord with 
the facts of physics and chemistry, a primary neces- 
sity to which our predecessors paid small heed. For 
them the unknown was a rich mine of possibilities on 
which they could freely draw. For us it is rather 
an impenetrable mountain out of which the truth can 
be chipped in rare and isolated fragments. Of the 
physics and chemistry of life we know next to nothing. 
Somehow the characters of living things are bound 
up in properties of colloids, and are largely determined 
by the chemical powers of enzymes, but the study of 
these classes of matter have only just begun. Living 
things are found by a simple experiment to have 
powers undreamt of, and who knows what may be 
behind ? 
Naturally we turn aside from generalities. It is no 
time to discuss the origin of the Mollusca or of Dico- 
tvledons, while we are not even sure how it came to 
pass that Primula obconica has in twenty-five years 
produced its abundant new forms almost under our 
eyes. Knowledge of heredity has so reacted on our 
conceptions of variation that very competent men are 
even denying that variation in the old sense is a 
genuine occurrence at all. Variation is postulated as 
the basis of all evolutionary change. Do we then as 
a matter of fact find in the world about us variations 
occurring of such a kind as to warrant faith in a 
4 I take the following from the abstract of a recent Croonian Lecture ‘ On 
the Origin of Mammals” deliyered to the Royal Society :—‘‘ In Upper 
Triassic times the larger Cynodonts preyed upon the large Anomodont, 
Kannemeyeria, and carried on their existence so long as these Anomodonts 
survived, but died out with them about the end of the Trias or in Rhzetic 
times. The small Cynodonts, having neither small Anomodonts nor small 
Cotylosaurs to feed on, were forced to hunt the very active long-limbed 
‘Thecodonts. The greatly increased activity brought about that senes of 
changes which formed the mammals—the flexible skin with hair, the four- 
chambered heart and warm blood, the locse jaw with teeth for mastication, 
an increas d development of tactile sensation and a great increase of cere- 
brum. Not improbably the attacks of the newly-evolved Cynodont or 
mammalian type brought about a corresponding evolution in the Pseudo- 
suchian Thecodonts which ultimately resulted in the formation of Dinosaurs 
and Birds.” Broom, R., Proc. Roy. Soc., B., 87, p. 88. 
NO: 2338, V@E.)03)| 
NATURE 
[AUGUST 20, 1914 
contemporary progressive evolution? Till lately most 
of us would have said “ yes’? without misgiving. We 
should have pointed, as Darwin did, to the immense 
range of diversity seen in many wild species, so com- 
monly that the difficulty is to define the types them- 
selves. Still more conclusive seemed the profusion 
of forms in the various domesticated animals and 
plants, most of them incapable of existing even for a 
generation in the wild state, and therefore fixed un- 
questionably by human selection. These, at least, for 
certain, are new forms, often distinct enough to pass 
for species, which have arisen by variation. But when 
analysis is applied to this mass of variation the matter 
wears a different aspect. Closely examined, what is 
the ‘‘ variability ’’ of wild species? What is the natural 
fact which is denoted by the statement that a given 
species exhibits much variation? Generally one of 
two things: either that the individuals collected in one 
locality differ among themselves; or perhaps more 
often that samples from separate localities differ from 
each other. As direct evidence of variation it is clearly 
to the first of these phenomena that we must have 
recourse—the heterogeneity of a population breeding 
together in one area. This heterogeneity may be in any 
degree, ranging from slight differences that systemat- 
ists would disregard, to a complex variability such as 
we find in some moths, where there isan abundance of 
varieties so distinct that many would be classified as 
specific forms but for the fact that all are freely breeding 
together. Naturalists formerly supposed that any of 
these varieties might be bred from any of the others. 
Just as the reader of novels is prepared to find that 
any kind of parents might have any kind of children 
in the course of the story, so was the evolutionist 
ready to believe that any pair of moths might produce 
any of the varieties included in the species. Genetic 
analysis has disposed of all these mistakes. We have 
no longer the smallest doubt that in all these examples 
the varieties stand in a regular descending order, and 
that they are simply terms in a series of combinations 
of factors separately transmitted, of which each may 
be present or absent. 
The appearance of contemporary variability proves 
to be an illusion. Variation from step to step in the 
series must occur either by the addition or by the loss 
of a factor. Now, of the origin of new forms by loss 
there seems to me to be fairly clear evidence, but of 
the contemporary acquisition of any new factor I see 
no satisfactory proof, though I admit there are rare 
examples which may be so interpreted. We are left 
with a picture of variation utterly different from that 
which we saw at first. Variation now stands out as 
a definite physiological event. We have done with the 
notion that Darwin came latterly to favour, that large 
differences can arise by accumulation of small differ- 
ences. Such small differences are often mere 
ephemeral effects of conditions of life, and as such 
are not transmissible; but even small differences, when 
truly genetic, are factorial like the larger ones, and 
there is not the slightest reason for supposing that 
they are capable of summation. As to the origin or 
source of these positive separable factors, we are 
without any indication or surmise. By their effects 
we know them. to be definite, as definite, say, as the 
organisms which produce diseases; but how they arise 
and how they come to take part in the composition 
of the living creature so that when present they are 
treated in cell-division as constituents of the germs, 
we cannot conjecture. 
It was a commonplace of evolutionary theory that 
at least the domestic animals have been developed 
from a few wild types. Their origin was supposed 
to present no difficulty. The various races of fowl, 
for instance, all came from Gallus bankiva, the Indian 
j 
