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Re 
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Nov. 17, 1870] 
NATURE 
49 


NATURAL SELECTION—MR. WALLACE’S 
REPLY TO MR. BENNETT 
M R. A. W. BENNETTS article entitled “ The Theory 
of Natural Selection from a Mathematical Point 
of View,” contairs several criticisms on my own writings, 
and touches on some points which have not yet been fully 
discussed. I propose, therefore, to reply to such of these 
as appear to be of sufficient importance. 
The first objection brought forward (and which had 
been already advanced by the Duke of Argyll) is, that 
the very title of Mr. Darwin’s celebrated work is a mis- 
nomer, and that the real “ origin of species” is that spon- 
taneous tendency to variation which has not yet been 
accounted for. Mr. Bennet further remarks, that through- 
out my volume of “ Essays” I appear to be unconscious 
that the theory I advocate does not go to the root of the 
matter; and this unconsciousness is not apparent only, 
for I maintain, and am prepared to prove, that the theory, 
if true, does go to the root of the question of the origin of 
species. The objection, which, trom its being so often 
quoted and now again brought forward, is evidently 
thought to be an important one, is founded ona mis- 
apprehension of the right meaning of words. It ignores 
the fact that the word “ species” denotes something more 
than “variety” or “individual.” A species is an organic 
form which, for periods of great and indefinite length as 
compared with the duration of human life, fluctuates only 
within narrow limits. But the “spontaneous tendency to 
variation ” is altogether antagonistic to such comparative 
stability, and would, if unchecked, entirely destroy all 
“species.” Abolish, if possible, selection and survival of 
the fittest, so that every spontaneous variation should sur- 
Vive in equal proportion with all others, and the result 
must inevitably be an endless variety of wnstable forms, 
no one of which would answer to what we mean by the 
word “species.” No other cause but selection, has yet 
been discovered capable of perpetuating and giving 
stability to some forms and causing the disappearance of 
hosts of others, and therefore Mr. Darwin’s book, if there 
is any truth in it atall, has a logical claim to its title. It 
shows how “ species,” or stable forms, are produced out 
of unstable spontaneous variations ; which is certainly to 
trace their “origin.” The distinction of “species” and 
“individual” is equally important. A horse or a number 
of horses, as such, do not constitute a species. It is the 
comparative Jermanence of the form as distinguished from 
the ass, quagga, zebra, tapir, camel, &c., that makes them 
one, Were there a mass of intermediate forms connect- 
ing all these animals by fine gradations, and hardly a 
dozen individuals alike—as would probably be the case 
had selection not acted—there might be a few horses, but 
there would be no such thing asa species of horse. That 
could only be produced by some power capable of elimi- 
nating intermediate forms as they arise, and preserving all 
of the true horse type, and such a power was first shown 
to exist by Mr. Darwin. The origin of varieties and of 
individuals is one thing, the origin of species another. 
Mr. Bennett next discusses the ph:nomena of “ mimic- 
ry,” and proposes to show, by mathematical calculations, 
that the effects could not be produced by natural selec- 
tion. But, at the very outset, he makes an important 
error, which seriously affects his subsequent reasonings ; 
for he leads his readers to understand that there is only 
one completely mimicking species of Leftalis, while the 
majority are of the normal white- butterfly type. The fact 
is, however, that but few species of Leféa/is retain the 
simple colouring of their allies the Pieridze, while the 
great majority are either coloured like the Heliconide, 
or show a considerable amount of colour or marking in 
that direction. He is also apparently unaware that some 
Heliconidz (/thomia eurimedia, for example) approximate 
in colour to the normal white and yellow species of 
Laptalis, and thus renders it much less difficult to under- 

stand how a sufficient amount of yariation in colour misht 
occur at a first step, to produce a resemblance which, 
viewed at some cons‘derable distance, would be de- 
ceptive, and therefore useful. 
We next come to the demonstration by means of figures 
and we here find still more serious errors, Mr. Ben. 
| nett says, that supposing a Leféalis may vary in twenty 
cifferent ways, one only being the direction required,— 
“the chance of any individual producing a descendant 
| which will take its place in the succeeding generation 
varying in the required direction, is },; the chance of 
this operation being repeated in the second generation is 
I . . . 
20? = i095 the chance of this occurring for ten successive 
generations is are or about one in ten billions ;” whence it 
is concluded that there are overwhelming chances against 
any progressive variation in the right direction ever taking 
place. But first, I do not admit the assumption that 
only one variation out of twenty would be in the right 
direction ; when it is remenibered how great is the variety 
of the Heliconidae, both in colour and marking, It seems 
more likely that one-fourth or one-third at least would 
help to approximate to some of them, and thus be useful, 
Taking, however, Mr. Bennett’s own figures, there are 
three great oversights in this one short sentence. The 
first is, that each Leféadés produces, not one only, but per- 
haps twenty or fifty offspring; the second is, that the 
right variation has, by the hypothesis, a greater chance of 
surviving than the rest ; and the third, that at each suc- 
ceeding generation the influence of heredity becomes 
more and more powerful, causing the chance of the right 
variation being reproduced to become greater and greater. 
Now with these three modifications the weight of the 
argument is entirely destroyed ; for, allowing the Leféadis 
to produce only twenty effspring (a small number for a 
butterfly), the chances become even that one out of the 
twenty varies in the right direction. But nineteen out of 
the twenty, on the average, are soon killed off by the 
various causes that keep down the population of the 
species, and the chances are very much in favour of that 
one surviving which, by the hypothesis, has varied in the 
right direction, It is not pretended that this one would 
survive always, or even on the average, but ina large 
number of cases it would certainly do so; and taking 
Mr. Bennett’s own estimate of a million individuals as the 
population of a rare species, we may fairly estimate that 
in a quarter, or say even in a tenth part of these, the sur- 
Viving offspring would possess the favourable variation. 
But now a new factor enters into the problem, of which 
Mr. Bennett takes no account. Those that have already 
varied tend to leave offspring varying in the same direc- 
tion as themselves ; and as these will all have an advan- 
tage, the offspring of the one-tenth will increase at the 
expense of those of the nine-tenths ; and this tendency 
being still more powerful in the third generation, with the 
additional advantage as the numbers increase of the 
chance of both parents being favourable varieties, we may 
fairly expect the favourable to have completely exter- 
minated the unfavourable variations, and to have firmly 
established themselves as a well-marked race. The enor- 
mous possible rapidity of multiplication, enabling a pair 
of individuals to produce millions in a few generations ; 
the survival of the fittest, giving to favourable variations— 
not their bare numerical chance, as Mr. Bennett supposes, 
but—a certainty in the long run of living at the expense of 
the rest ; and the powerful influence of heredity, which 
actually increases the ¢endency to produce the favourable 
variations with each succeeding generation,—are three of 
the main foundation-stones of the theory of natural selec- 
tion, yet all three are ignored in this attempted mathe- 
matical demonstration of its insufficiency. a 
There is one other point in the theory of the origin of 
“mimicry” that deserves notice. It is, that the modifica- 
tions leading to it are much more easy to explain than 
