66 
NATURE 
[Nov. 24, 1870 

admirable piece of special pleading, based on a skilful assump- 
tion of premisses which, to a careless or biassed observer, might 
seem indisputable. 
The tendency to variation is spoken of as something very mys- 
terious, of which no adequate account has ever yet been given. 
Yet the very simple explanation is no bad one, that where two 
parents are concerned in the production of any offspring, the 
product in part resembling each of the producers must of ne- 
cessity also in part differ from each of them. Between the parents 
themselves, Mr. Herbert Spencer has shown that differences of 
age and external circumstances would ensure the requisite want 
of resemblance in the absence of any other cause. 
‘« The rigid test of mathematical calculation” is then applied 
to the case of mimetic butterflies, with the view of showing that 
they could not have been produced simply according to the laws 
of variation, inheritance, and natural selection. In the applica- 
tion of this rigid test the very first step is a perfectly gratuitous 
assumption, ‘*that it would require, at the very lowest calcula- 
tion, 1,000 steps to enable the normal Zef/a/is to pass on its 
protective form.” Who is to prove that fifty differences would 
beinsufficient? An interval of athousand years might be granted 
for establishing each one of these variations. Suppose even 
50,000, instead of only 50 steps to be necessary, it is another 
gratuitous assumption that ‘‘the smallest change in the direc- 
tion of the /thomia, which we can conceive in any hypothesis to 
be beneficial to the Zeféalis, is at the very lowest one-fiftieth of 
the change required to produce perfect resemblance.” How 
small a difference must decide the choice made by a donkey 
placed equidistant between two bundles of hay! Certainly, then, 
a bird on the wing, having to choose amidst myriads of butter- 
flies, may be determined by an almost infinitesimal distinction, 
Further, though the whole change may be produced by an im- 
mense number of small changes, it is not necessary to suppose 
that all the changes will be equally small. It is merely begging 
the question to assume that the first change could not possibly be 
large enough to be of any use. And if it may be of use, the 
whole mathematical calculation, based on its being useless, breaks 
down from the beginning. Again, since the Zeffa/is may have 
spent 1,000,000 years in arriving at its present likeness to the 
present Z¢homia, it is impossible to assert that the normal forms 
of the two butterflies were as wide apart at the beginning of 
that period as they are at present. ‘The mimicry having once 
set in, might be retained by parallel variations. This, indeed, 
cannot fail to be the case, if the protection is to bea lasting one ; 
for when the /thomia varies in outward appearance, unless the 
Leptalis varies in the same direction, the resemblance will be lost. 
This progressive mimicry would be more valuable than an imi- 
tation in which no changes occurred, since the enemies of a mi- 
metic species would in time become aware of a fraud which had 
no variations at its command, as birds are said now-a-days to 
pounce without hesitation upon caterpillars which very much 
resemble twigs. Even ‘‘a rough imitation” may be useful in 
the first instance, and yet when hostile eyes have long been exer- 
cised, and have acquired greater and greater sharpness, finally 
nothing less than adso/ute identity of appearance may be thoroughly 
effective. Thus the perfecting of the resemblance will be no 
‘mere freak of Nature,” nor shall we be ‘‘landed in the di- 
lemma that the /as¢ stages are comparatively useless” in this 
procedure. 
The array of figures brought forward to prove that the Zep- 
talis could not have made twenty steps of variation in the direc- 
tion of the Zéhomia by chance, would be much to the purpose if 
any exponent of the theory of Natural Selection had ever argued 
or supposed that it could. The calculation takes it for granted 
that the theory is erroneous, instead of proving it to be in error. 
Upon this assumption, it might have been put far more strongly, 
only that a stronger way of putting it would have borne on the 
face of it the suspicion of some inherent fallacy. It begins by 
supposing that there are “‘twenty different ways in which a 
Leptalis may vary, only one of these being in the direction ulti- 
mately required ;” it might quite as truthfully, or even more so, 
have said a thousand instead of twenty, and then the second step 
would have given the chance as only one in a million, instead of 
one in four hundred. But while the theory of Natural Selec- 
tion speaks of numerous minute useful variations, Mr. Bennett 
will not allow that combination of terms. Let them be numerous 
and minute, if you will, he says, but if small they cannot be use- 
ful, if useful they cannot be small. He claims to have Mr. 
Darwin’s own word for it, that a large variation would not be 
permanent, as though Mr. Darwin had said, “living creatures 

have come to be what they are by successive useful deviations. 
of structure permanently propagated, but no large deviations are 
permanent, and no small ones are useful.” It is quite obvious 
that in the use of relative terms, such as great and small, Mr. 
Darwin neither intended to stultify himself nor has done so. A 
thing may be large enough to be useful without being large 
as compared with something twenty times its own size; and a 
man may be said to have a huge brain in a very small body, 
although the body in solid content far exceeds the brain. When 
Mr. Darwin says that ‘‘ Natural Selection always acts with ex- 
treme slowness,” he does not imply that its steps must therefore 
be ‘so numerous as to be too small to confer any advantage. 
This would be a contradiction in terms. But the steps may be 
exceedingly small notwithstanding, and also sometimes separated 
by enormous intervals of time from one another. 
In introducing his own explanation of things, Mr. Bennett 
affirms that ‘‘ resemblances, and resemblances of the most 
wonderful and perfect kind” in the vegetable kingdom, ‘“‘ are 
in no sense mimetic or protective.” This may be so, but it can 
hardly be said to be proved. When he speaks of ‘‘ man’s 
reason” haying ‘‘assisted him so to modify his body as to 
adapt himself to the circumstances with which he is surrounded,” 
and suggests that the instinct of animals may have assisted them 
also to modify their bodies by slow and gradual degrees to the 
same purpose, it is difficult to imagine the process intended, and 
still more difficult to see how ‘‘the slow and gradual degrees ” 
will escape the rigid test of mathematical calculation which Mr, 
Bennett has elsewhere applied ; for if the steps are great they 
ought not to be permanent, and if small they ought not to be 
useful. A theory which makes it possible for a bee to ‘‘ modify 
its proboscis” by instinct, or for a man to treat his nose in the 
same manner by reason, seems harder of digestion than the Dar- 
winian. Tuomas R. R. STEBBING 
Torquay, Nov, 12 

Mr. BENNETT‘, in his very able paper read before the British 
Association at Liverpool, and published in NATURE of the roth 
November, calls in question the explanation given by the theory 
of Natural Selection of the various instances of mimicry found in 
the animal kingdom. 
He bases his argument principally on the fact that the altera- 
tions in the early stages being useless to the animal would not 
be preserved, and that these changes must be very slow. — 
He assumes that to enable the normal Zef/a/zs to imitate a 
species of Zthomia, it may be considered to have gone through at 
least 1,000 stages, and that no change less than one-fiftieth of the 
whole alteration effected would be of any use to the insect. He 
gives us no information as to how he arrives at these figures, and 
we are left with the idea that they are selected principally because 
they are what are called ‘‘ round numbers,” and are more easily 
dealt with in the calculation which he gives us. 
Now I think that the number of stages which Mr. Bennett con- 
siders it necessary for a Zeffalis to pass through so as to mimic 
an Jthomia is vastly too great: I,000 stages means at least 
1,000 years. 
Let us look at the alteration which frequently takes place in 
the colouring of a butterfly, possibly in one generation, as shown 
by varieties of which sometimes only solitary specimens are 
known, figured in Newman’s work on English Butterflies, I 
need only refer your readers to the figures of varieties of 4patura 
iris, Epinephele janira, Limenitis sibylla, Melitea athalia. 
Now can it be contended that it required 1,000 of such stages to 
effect the alteration ? 
If any of these variations happened to be useful, there seems 
no reason for supposing that one stage might not make much 
more than ;; of the alteration, which Mr. Bennett lays down as 
being the least which would be useful, and which I agree with 
him in considering much too small. Why might not one stage 
make one-fourth or one-sixth of the alteration required ? 
Mr. Darwin quotes a passage in his work on Natural Selec- 
tion (page 32) from Sir John Sebright with regard to pigeons, in 
which he says that it takes three years to produce a given feather, 
but six years to make a head and beak. If the bony structure 
of an animal so far above a butterfly can be altered in six years, 
we surely do not require more than that time to effect an alteration 
in the colour of a butterfly’s wing. 
Mr. Bennett states that the early stages of the alteration would 
be useless to the insect; every one, I think, will grant this, when 
each stage is only one-thousandth of the whole, but not if it be 
= 
