362 
NATURE 
_ [August 19, 1886 
intended to describe the real phenomena, may I ask you to allow 
me a few lines of space for the following : 
The real sun was surrounded at a short distance by a halo or 
rainbow circle of great beauty, with a mock sun of the same 
apparent magnitude and brightness on the right and left ; and 
portially formed suns above and below the ring: all of them 
being slightly opalescent. From the two perfected suns pro- 
ceeded cones of intense light, about 3 diameters of the sun in 
length, and with their apices pointing east and west. ‘These 
were rather more opalescent than the mock suns from which they 
seemed to originate. A second rainbow ring at a considerable 
distance outside of these extended to the zenith. The period of 
greatest beauty and brightness, when they were as rich in colour- 
ing as areal rainbow, lasted about 5 minutes. I was able to 
watch the whole of the phenomena from a little after 4 to nearly 
6 o'clock. Ropert H. F, Rippon 
Jasper Road, Upper Norwood 
PHYSIOLOGICAL SELECTION: AN ADDI- 
TIONAL SUGGESTION ON THE ORIGIN OF 
SY ZV HEITOR 
III. 
ARGUMENT from the Prevention of Iniercrossing.— 
This argument is the same from whatever cause the 
prevention of intercrossing may arise. Where intererossing 
is prevented by geographical barriers or by migration, it is 
more easy to prove the evolution of new species as a con- 
sequence than it is when intercrossing is prevented by 
physiological barriers; for in the latter case the older 
and the newer forms will probably continue to occupy the 
same area, and then there will be no independent evidence 
to show that the severance between them was due to 
the prevention of intercrossing. Nevertheless, all the 
evidence I have of the large part that geographical 
barriers have played in the evolution of species by pre- 
venting intercrossing with parent forms goes to show the 
probable importance of physiological barriers when acting 
in the same way. Henceit will be better to postpone this 
line of argument in favour of physiological selection until 
the appearance of my next paper, where I shall hope to 
show, from evidence furnished by the geographical distri- 
bution of species, how predominant a part the prevention 
of intercrossing has played in the evolution of species. 
Here, therefore, I will merely remark that wherever inter- 
‘crossing with parent forms is prevented, in the proportion 
‘that it is prevented a better opportunity is given to 
natural selection for seizing upon any beneficial varia- 
tions that may happen to arise. On this account physio- 
logical selection probably lends important aid to natural 
selection, thus becoming indirectly instrumental in the 
evolution of useful as well as of useless structures. 
There is also another respect in which these two kinds 
of selection probably co-operate. For Mr. Darwin shows 
that “it would be clearly advantageous to two varieties, 
or incipient species, if they could’be kept from blending, 
on the same principle that, when man is selecting at the 
same time two varieties, it is necessary that he should 
keep them separate.” But he proceeds to show that this 
advantage cannot be conferred by natural selection, and 
hence that the sterility which is so generally character- 
istic of species cannot be attributed to this agency. We 
have, however, just seen that this sterility is in all likeli- 
hood due to physiological selection; and therefore, if it 
be true, as Mr. Darwin thought, that “it would profit an 
incipient species if it were rendered in some slight degree 
sterile with its parent form,” physiological selection and 
natural selection may mutually assist one another. For, 
although the benefit of this sterility could not have been 
initially conferred by natural selection, yet when it once 
arises from an independent variation in the reproductive 
system, there is no reason why it should not forthwith be 
favoured by natural selection, just as is the case with 
advantageous variations in general. 
t Abstract of a Paper read before the Linnean Society on May 6, by 
George J. Romanes, M.A., LL.D., F.R.S. &c. Continued from p. 340. 
‘structures are sometimes of the most curious and complex 
an eee Ne ert 
Feeling how grave a difficulty was presented to his 
theory of the origin of species by the general sterility of 
species, Mr. Darwin was extremely anxious to find some 
way in which natural selection might be seen to have 
brought about this result. Had it occurred to him that 
this result was probably nothing more than the necessary 
expression of a particular kind of variation on the part of 
the reproductive system, I cannot doubt that he would 
have felt the theory of natural selection to have been 
relieved of one of its greatest disabilities. 
Argument from the Inutility of Specific Differences — 
After what has already been said on this subject, I will 
here only deal with one question, namely, Why is it that 
apparently useless structures occur in such profusion 
among species, in much less profusion among genera, and 
scarcely at all among families, orders, and classes? It 
may be answered that the points wherein species differ 
from species are usually points of smaller detail than 
those which distinguish genera, families, &c., and thus 
may well actually be as a rule less useful, although still 
not absolutely useless : natural selection, it may be urged, 
is better able than is the naturalist to diagnose utility. 
But here again we have a most unwarranted appeal to the 
argument from ignorance ; whereas, according to my view, 
it is quite intelligible that when a varietal form is differ- 
entiated from its parent form by the bar of sterility, isola- 
tion, or migration, any little meaningless peculiarities of 
structure (or of instinct?) should at first be allowed to 
arise, but should eventually be eliminated as so much 
surplusage in the struggle for existence, by economy of 
growth, or even by independent variation when undirected 
by natural selection. A greater or less time would in — 
different cases be required to effect this reduction, and 
thus we can understand why they are sometimes allowed 
to persist into genera, but rarely into families. : 
Again, if apparently useless specific characters (whether 
these be new structures or modifications of old ones, — 
slight changes in form, colour, and so forth) are thus 
regarded as really useless, we should expect that they 
ought to be of a kind which do not impose much physio- 
logical tax upon the organism, since otherwise natural — 
selection would not have allowed them to become so 
much as specific characters. Well, I have applied this 
test, and find it is a most general rule that specific 
characters the utility of which cannot be perceived are 
such as do not impose any considerable demand for 
nourishment: either on account of their small size or 
of their organically inexpensive material, they do not im- 
pose much tax upon the organism. Now it is obvious 
that there can be no connection between utility as dis- 
guised and smallness of size or inexpensiveness of 
material ; while it is no less obvious that there is a close 
connection between these things and a real inutility. 
Lastly, our domesticated varieties ‘occasionally exhibit 
well-marked and more or less constant characters of a 
useless kind. Here there can scarcely be any question 
about the genuineness of the inutility, seeing that the 
characters have arisen only under domestication, or in 
the absence of any struggle for existence. Yet these 
morphology—even more so than innumerable apparently 
useless structures in the case of natural species? 
Argument from Divergence of Character —Any theory 
of the origin of species in the way of descent must be 
prepared with an answer to the question, Why have 
species zultiplied? Why have they not simply become 
transmuted in linear series instead of ramifying into 
branches? This question Mr. Darwin seeks to answer 
* For instances of useless instincts see Mr. Darwin’s posthumous essay 
published in my “Mental Evolution in Animals.” It is suggestive in the 
present connection that, just like useless structures, useless instincts, so far 
as I can find, only occur in species and genera: never in families, orders, or 
classes. 
* For a good instance of this see ‘‘ Variation of Plants and Animals under 
Domestication,” vol. i. pp. 78-79. 
