Sept. 2, 1886] 
archipelagoes are treated with equal thoroughness, and the 
work is provided with a map of the Indian Ocean, an 
index, and numerous well-executed woodcuts. 
LETTERS TO THE EDITOR 
(Zhe Editor does not hold himself responsible for opinions ex- 
pressed by his correspondents, Neither can he undertake to 
return, or to correspond with the writers of, rejected manu- 
scripts. No notice is taken of anonymous communications. 
[The Editor urgently requests correspondents to keep their letters 
as short as possible. The pressure on his space ts so great 
that it ts impossible otherwise to insure the appearance even 
of communications containing interesting and novel facts.) 
Physiological Selection and the Origin of Species 
In the Yournal of the Linnean Society (Zoology, No. 115, 
1886, p. 350, footnote) Mr. Romanes says: ‘‘I cannot find that 
any previous writer has alluded to the principle which it is the 
object of the present paper to enunciate, and which is explained 
in the succeeding paragraphs.” 
But in the fourth edition of the ‘* Origin of Species” (1866), 
p. 311,! the following passage occurs, in which the main idea 
of ‘* physiological selection” is clearly alluded to. 
“*Tt may be admitted, on the principle above explained, that 
it would profit an incipient species if it were rendered in some 
slight degree sterile when crossed with its parent-form or with 
some other variety ; for thus fewer bastardised and deteriorated 
offspring would be produced to commingle their blood with the 
newly-forming variety.” 
The author then goes on to show that, as he believed, this 
kind of sterility could not be increased by natural selection—a 
discussion with which Iam not now concerned. I have other 
evidence to show that my father was familiar with the principle 
of physiological selection, and, moreover, that he did not regard 
it with any great favour. 
In Mr. Belt’s ‘* Naturalist in Nicaragua” (1874), a suggestion 
is made, identical with that of Mr. Romanes in the Linnean 
Journal, Mr. Belt says (p. 207) :—‘‘ The varieties that arise 
can seldom be separated from the parent form and from other 
varieties until they vary also in the elements of reproduction. 
. .. «As long as varieties interbreed together and with the 
parent form, it does not seem possible that a new species could 
be formed by natural selection, excepting in cases of geographical 
isolation. All the individuals might vary in some one direction, but 
they could not split up into distinct species whilst they occupied 
the same area and interbred without difficulty. Before a variety 
can become permanent, it must either be separated from the 
others or have acquired some disinclination or inability to 
interbreed with them. As long as they interbreed together, 
the possible divergence is kept within narrow limits, 
but whenever a variety is produced the individuals of 
which have a partiality for interbreeding, and some amount 
of sterility when crossed with another form, the tie that bound 
it to the central stock is loosened, and the foundation is laid for 
the formation of a new species. Further divergence would be 
unchecked, or only slightly checked, and the elements of repro- 
duction having begun to vary, would probably continue to 
diverge from the parent form, for Darwin has shown that any 
organ in which a species has begun to vary is liable to further 
change in the same direction. ‘Thus one of the best tests of the 
Specific difference of two allied forms living together is their 
sterility when crossed, and nearly allied species separated by 
geographical barriers are more likely to interbreed than those 
inhabiting the same area.” 
In my copy of Belt’s book the words “‘ No, No,” are pencilled 
in my father’s handwriting on the margin, opposite the sentence 
“All the individuals might vary in some one direction, but they 
could not split up into distinct species whilst they occupied the 
same area and interbred without difficulty.” 
Cambridge, August 27 FRANCIS DARWIN 
NEITHER Mr. Galton nor Mr. Meldola have had time or 
opportunity to consult my original paper before writing their 
comments on the Nature abstract. I will, therefore, consider 
1 A corresponding but not identical passage occurs in the sixth edition, 
P. 247. 
NATURE 
407 
those of their remarks which have been anticipated in the 
paper. E 
Mr. Galton writes :—‘‘It has long seemed to me that the 
primary characteristic of a variety resides in the fact that the 
individuals who compose it do not, as a rule, cave to mate with 
those who are outside their pale, but form through their own 
sexual inclinations a caste by themselves.” Now, I have fully 
recognised this principle as one among several others which is 
accessory to, although independent of, physiological selec- 
tion: see L.S. paper, p. 377, where also reference is given to 
the ‘‘ Origin of Species,” showing that this factor was likewise 
recognised by Mr. Darwin as one of importance in the preven- 
tion of intercrossing. But, inasmuch as this factor—which may 
be called psychological selection—can only apply to the case of 
the Vertebrata,! I am disposed to think that it is of much less 
general importance than the other factors which I have men- 
tioned as accessory to physiological selection, and which, taken 
altogether, furnish a complete theoretical explanation of the fact 
that sterility between natural species is not invariably absolute, 
but occurs in all degrees. For, ‘‘in all these cases where the 
principles of physiological selection have been in any degree 
accidentally assisted by other conditions, a correspondingly less 
degree of variation in the reproductive system would have been 
needed to differentiate the species” (p. 377). 
Thus far, therefore, Mr. Galton is really in full agreement 
with me. But he goes on to say :—‘‘If a variety should arise 
in the way supposed by Mr. Romanes, merely because its mem- 
bers were more or less infertile with others sprung from the 
same stock, we should find numerous cases in which members of 
the variety consorted with outsiders.” But how can we possibly 
know that such is not the case? If my theory is true, it must 
follow, as Mr. Galton says, that such unions would be more 
or less sterile, and, as this sterility is itself the only variation 
which my theory supposes to have arisen zz the first instance, 
ex /iypo'hesi we can have no means of observing whether or not 
the individuals which present this variation ‘‘ consort with out- 
siders,” or with those individuals which do not present it. 
Lastly, in as far as it is true that “we hardly ever observe pair- 
ings between animals of different varieties when living at large 
in the same or contiguous districts,” the fact in no way makes 
against my theory of physiological selection: it only serves to 
| supplement this theory, in the case of higher animals, by what 
I regard with Mr. Galton as the proved facts of psychological 
selection. 
The letter by Mr. Meldola is a masterpiece of Darwinian 
thinking, and on this account I am glad to find myself much 
more in agreement with him than he appears to suppose. For 
when he reads my full paper he will see that I have taken pre- 
cisely the same view upon natural selection as a possible cause 
—or, rather, accessory promoter—of specific sterility as that 
to the statement of which the larger part of his letter is devoted. 
I may remark, however, that of all parts of my paper I regard 
this as the most speculative and least secure. And this, first, 
because Mr. Darwin himself, after profound meditation upon 
the subject, came to the conclusion that natural selection could 
not operate so as to induce sterility ; and, next, because the 
supposition that it does so operate involves one of*the most 
difficult and complex questions in the whole philosophy of 
evolution—namely, whether it is possible for natural selection to 
modify an entire ¢yfe without reference to benefit of its con- 
stituent zzdividuals. Now, although for reasons which need 
not here be detailed, I have been led, like Mr. Meldola, to 
take a different view from that of Mr. Darwin, and to conclude 
that natural selection may benefit the type without reference to 
the individual, still I regard this conclusion as so highly specu- 
lative that I am glad to think the much more certain theory of 
physiological selection is not vitally affected either by its accept: 
ance or its rejection. If it is true that natural selection may be 
able to modify an organic type (as my critic and myself agree in 
arguing, the type in this case being a variety) by conferring on 
it the benefit of sterility with its parent form, notwithstanding 
that this cannot be effected through benefit conferred on any of 
the constituent individuals, then all we have to say in the 
present connection is that natural selection is probably one of 
the many other causes which lead to physiological selection. 
1 This, at least, is what I state in the paper. Mr. Galton, however, sug- 
gests that the principle may be extended even to plants, through “‘the 
selective appetites of the insects which carry the pollen.” This suggestion 
is unquestionably original, and bears the stamp of its author's ingenious 
mind. Moreover, considerable probability is, I think, lent to the suggestion 
by the observations of Mr. Bennett and others on individual insects selecting 
similarly coloured flowers on which to feed (see Yourn. L.S., 1883). 
