86 
NATURE 
[Dec. 1, 1870 

and that it is not difficult to explain, yet I do not feel bound to 
explain every supposed fact as if it were a well-estabiislied one. 
As to the ‘parallelism of the development of protective resemb- 
lance and of instinct in the animal world,” which I am also 
asked to explain, I deny that it has been proved to exist. 
In conclusion, I will o» erve that the theory of Natural Selec- 
tion, and its subordinate theory, Mimicry—have now been so 
fully developed by Mr. Darwin, Mr. Bates, Mr. Trimen, and 
myself, that I conceive it to be a full and sufficient answer to any 
opponent if we c n show that his particular objections are un- 
sound. ‘This, I believe, I have done in the case of Mr. Bernett, 
although I am sorry to find that he cannot see it, and it is there- 
fore unnecessary to go fully into the collateral points on which 
he has touched, and which have already been sufficiently ex- 
plained by Mr, Darwin or myself. 

ALFRED R. WALLACE 
I am forcibly reminded of Pope’s lines, 
A little knowledge is a dangerous thing: 
Drink deep, or taste not, the Pierian spring, 
by the argument used by Mr. Bennett in the P.S. to his letter 
in NATuRE, of the 24th November, in which he says, after 
quoting a passage from a paper by Mr. Jenner Weir: ‘‘ Here at 
least it would seem as if zmferfect mimicry was anything but 
beneficial to the individual ; how can the principle of natural 
selection account for its propagation in these instances?” He 
considers that a little mimicry isa dangerous thing. I would 
rather agree with Lord Brougham in his remark on the above 
lines, that as a little knowledge is better than greatignorance, so 
a little mimicry is better than great dissemblance. 
But the case referred to by Mr. Jenner Weir is plain, and the 
argument, instead of being against the theory of natural selec- 
tion, is really in its favour. 
Some of the larvze in question, for some reason of which we 
are unaware, are not so palatable to birds, and they, therefore, 
are not eaten by them to the same extent. These larve have 
aot so much need of the aid of protective resemblance, and 
indeed their hair, spines, and gay colouring are advantageous to 
them instead of a drawback. The smooth-skinned larvee require 
the aid of protective resemblance for their preservation, but no 
one would fora moment expect that because an insect has a 
protective resemblance to the place on which it rests, that every 
individual is to escape destruction by its enemies. 
Mr. Bennett again asks for an explanation of the tendency of 
the South American Zeftalide to resemble /thomie. I think 
the reason is clear. Mr. Bates, in his paper, read before the 
Linnean Society in 1862 (Trans., vol. 23), states that the Zef/a- 
fide are exceedingly rare compared to the Aeliconide, and that 
the proportion is about I to 1,000, and also that none of the 
Leplalide ave found in any other locality than those of the 
H-liconide they mimic. From this I should judge that the 
Leptalide cannot make head against their enemies, and require 
the assistance of mimicking some better protected_species to be 
able to maintain itself. 
November 25 S. N. CARVALHO, JuN. 
ProressoR HUXLEy has referred Mr. Bennett to the highest 
authority for an answer to his reasoning on a difficulty in the 
theory of natural selection. Meanwhile, Mr. Wallace has replied 
on his own account. Upon the biological question I do not pre- 
sume to touch, but I wish to say a word upon the mathematical 
one, especially as I cannot think Mr. Wallace has really met this 
part of the argument. 
Mr. Bennett’s argument is shoztly this. A modification must 
be advantageous before natural selection can take hold of it. In 
order to be advantageous, it must not be too small ; it must be 
so great as to be attainable only in the course of many genera- 
tions, during which, in the absence of natural selection, we must 
see whether chance will carry us over the ground. As an ex- 
treme concession, he supposes that an advantageous amount of 
change might be accumulated in twenty steps; and, assuming 
that the required direction of change is only one out of twenty 
directions equally probable, he easily shows it to be violently im- 
probable that a stationary population of one million should pro- 
duce a single instance of even ten such steps in successive gene- 
rations. 
But why is it necessary to suppose the steps made in succes- 
sive generations ? 


within reasonable time, it may surely be immaterial what inter- 
vals of merely unprogressive variation may elapse between them. 
In 200 generations, the first, fifteenth, fiftieth, for instance, and 
seventeen more, might make steps in the right direction, and all 
the rest might make steps in some or all of the other nineteen 
possible directions. Ten would in fact be the most probable 
number of steps in the right direction, and it would be about an 
even chance that there were ten at least. 
However, as soon as we suppose steps in other directions, we 
must allow for the possibility of steps which shall actually reverse 
such progress as might be made in the right direction. If one 
change out of twenty equally likely is in the right direction, there 
will be on an average one in the opposite direction, and eighteen 
in indifferent directions. If we assumed that, in 200 generations, 
180 were neutral, while twenty made steps forward or steps back- 
ward, these twenty wight be all forward, and the chance that 
they were so would be one in 2”, or one in little more than a 
million. Generally, the number of neutral steps would be a little 
more or a little less than 180, and if we allow for this the re- 
sulting chance will be considerably increased. Several instances 
would probably be produced bya population of a million; and 
I presume it is easy to allow much more than 200 generations of 
butterflies. 
Noy. 23 C. J. Monro 

Dr. Nicholson’s ‘‘ Zoology” 
I NoTICE in NATURE for Oct. 20, a review by Mr. E. Ray 
Lankester, of a Manual of Zoology recently published by me, 
and I crave a small portion of your space to say a few words 
thereon. Upon Mr. Lankester’s zoological strictures on my 
work I will not enter, partly because the public verdict on the 
merits of my work has already been very emphatically and deci- 
sively expressed ; partly because the sins laid to my charge are 
chiefly of omission and not of commission, and are, therefore, 
more or less inevitable in a work of such limited compass; and 
partly because it must be patent to everyone how much more 
admirably the work, unfortunately left to me, would have been 
discharged by Mr. Lankester himself. 
In the matter of Greek, however, Mr. Lankester really must 
excuse me if I decline to bow to his superior knowledge. Iam 
well aware that he probably entertains a fresher recollection of 
his school days than I can boast of, and I might, therefore, with- 
out shame, have pleaded guilty to some obliviousness of Greek 
roots. Mr. Lankester, however, has been singularly unlucky 
in the point of attack chosen by him. He takes upon himself 
to condemn the whole of the glossary to my work, because he 
finds the twelfth word of the same (‘‘actinomeres”) derived from 
the Greek word ai, and he is good enough to add the informa- 
tion that ‘‘there is no such Greek word as atin.” Now, any 
decent lexicon would have informed Mr. Lankester that aééiz is 
not only good Greek, but that it is the original form of the word, 
and that aktis was employed for the first time by Pindar, not, 
therefore, till about 450 B.c. 
In conclusion, if 1 may be permitted to make a suggestion, I 
would recommend Mr. Lankester, in his capacity as critic and 
appraiser of the work of other men, not to judge in future of 
the value of a haystack by the first straw that he may happen 
to pull out of it; or, if he must do this, to be very sure before 
giving his opinion to the public, that it z a straw that he has 
succeeded in laying hold of. 
Newhaven, Edinburgh 'H. ALLEYNE NICHOLSON 

Dr. NICHOLSON’s extraordinary assertions as to the supposed 
word ‘‘aktin” really demand no serious discussion, which, in- 
deed, would be out of place in NATURE. A reference to Liddell 
and Scott’s Lexicon will conclusively demonstrate to any person 
interested in the matter that he is entirely wrong. The follow- 
ing additional blunders in Dr. Nicholson’s giossary will enable 
your readers more fully to judge of his accuracy, and it will re- 
quire considerable boldness to attempt to justify them by refer- 
ence to imaginary archaic forms :—1. In several places we find 
Dr. Nicholson giving ‘‘poda” as the Greek for “feet,” a gross 
grammatical fault. 2. “ Pseudos” is given as the adjective cor- 
responding to the English word “false.” 3. ‘‘Enchuma” is 
said to be a Greek word meaning “tissue.” It has not this 
meaning. Dr. Nicholson’s mistake arises from ignorance of the 
origin of the signification of the word “parenchyma.” 4. ‘‘ Laima” 
is given in several places in the glossary for ‘‘ throat,”’ in place of 
Provided that the required number are made | “‘laimos.” 
