Oct. 28, 1886] 
LETTERS TO THE EDITOR 
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On the Connection between Chemical Constitution 
and Physiological Action 
IN his letter to NATURE last week (p. 594), Dr. Blake con- 
siders that I have not only misunderstood the scope of his 
experiments, but have been led into error on account of my 
having no definite idea of the meaning of the term chemical 
constitution, which he thinks I have evidently confounded with 
that of chemical composition. 
In regard to the first of these points, I shall be very sorry if, 
by mishap, I have not rightly understood, or have failed to 
appreciate at their true value, Dr. Blake’s experiments (most of 
which were published before I was born), for I regard him as 
a true pioneer in the field of pharmacology. 
The scope of Dr. Blake’s researches, as defined by himself in 
the Report of the British Association for the Advancement of 
Science for 1846, was ‘‘ fully to establish the law of the analogous 
action of isomorphous substances.”’ 
I should no doubt have described Dr. Blake’s researches 
more correctly had I used the word ‘‘ isomorphous ” instead of 
translating it into popular language, for my translation un- 
doubtedly does not give the full meaning of the word; but my 
whole address was an attempt to make a difficult subject as 
popular as I could, and I thought that I had sufficiently acknow- 
ledged Dr. Blake’s priority by observing that the present epoch 
of pharmacology might be dated from his researches, although 
it was those of Crum Brown and Fraser which fairly started 
pharmacological investigation in a new direction. Perhaps Dr. 
Blake will be inclined to regard my shortcomings in regard to 
him more leniently if he will read over my address again, for, 
if he does so, I think he will see that if on my part I have 
failed to give him due credit, he on his part has completely 
misunderstood the whole drift of my address, which was to show 
the importance of chemical constitution as distinguished from 
chemical composition. T. LAUDER BRUNTON 
The Origin of Species 
Ir has already been pointed out by Mr. Evershed that the 
‘* physiological selection ” of Dr, Romanes is identical with the 
theory outlined by me nearly two years ago in these pages 
(vol. xxxi. p. 4). As all the objections which have been raised 
apply equally to my theory, I may perhaps be allowed to give 
my answer to some of them; it will probably differ in some 
points from that promised by Dr. Romanes in the Fortnightly. 
I quite agree with Mr. Wallace (in the Fortnightly) that it is 
only among the group of animals which have at least one 
common parent that the corresponding variations of the sexual 
organs which are required for physiological insularity would be 
likely to occur. But when he maintains that not more than two 
or three of such a group would reach maturity, and that there- 
fore they would soon die out, he seems to me to forget that it is 
only on the average that the number would be so small. Many 
groups would be small, and would die out; exceptional families 
would be more numerous and more lucky ; just as we can all 
point to human families where twelve or more children have 
reached maturity, though the average number of those who do 
so is under three in a family. 
The survivors, more or less numerous, would generally not be 
scattered far from their common birthplace, so that their chance 
of finding one another would not be very small, especially if the 
sexual instinct was correspondingly modified, and this might 
well be the case from what we know of the connection between 
the psychical and physiological parts of the reproductive function. 
This presupposes some difference of smell, form, colour, &c., to 
enable an animal to distinguish those of its own family from the 
rest of the species, but this probably exists between any two 
animals. 
They might thus be under no great disadvantage compared 
with the parent species, and they would have a counterbalancing 
advantage in the much greater adaptability to circumstances 
NATURE 
617 
which a small group possesses. Any useful variation occurring 
ina large group, if not swamped by the effect of interbreeding 
with a large number of unimproved forms, must take many 
generations to modify the whole mass; while a similar useful 
variation occurring in one member of a small and physiologic- 
ally isolated group could modify the whole group ina few gener- 
ations. The existence of a six-fingered man in England would 
not appreciably modify the inhabitants in a thousand years, even 
if it was a slight advantage to have six fingers ; while if a six- 
fingered man was introduced into an island with five other in- 
habitants, a fair proportion of the population would probably 
be six-fingered in three generations. 
It is perhaps worth pointing out that the curious connection 
between colour and fertility, in which Mr. Wallace seeks for the 
explanation of the sterility of species, follows at once as a corol- 
lary from the doctrine of physiological selection. For, apart 
from any special modification of the sexual instinct, all animals 
seem to prefer to breed with those of their own colour, and 
hence avy change of colour in the isolated family would be an 
advantage, and would indeed remove the one disadvantage 
under which such a family lies. So a change of colour, other- 
wise useless, would in such cases be preserved, and be found 
accompanying sterility with the parent species. 
Another of Mr. Wallace’s objections seems to me a strong 
argument in favour of my (and Dr. Romanes’s) theory. He says 
that some animals, not only of different species but of different 
genera, can produce hybrids, and he instances the pheasant and 
black grouse. Now this is just what we ought to find on our 
theory, and ought not to find on any other. If either structural 
divergence or divergence in colour produces infertility, then the 
pheasant and the black grouse should be sterile, since they differ 
more, both in structure and colour, than many sterile pairs. But 
if species are produced sometimes by physiological isolation, but 
sometimes by other causes, such as geographical isolation, spon- 
taneous distaste (not disability) for pairing, or even unaided 
natural selection, then those species which have been produced 
by aid of any of these latter processes will be fertile in spite of 
any ordinary amount of divergence, since nothing has occurred 
to render them otherwise ; while those which have been formed 
by physiological isolation will be sterile even though they have 
hardly diverged at all. We cannot tell, without assuming what 
I am trying to prove, what form of isolation has been at work, 
except in the case of island species; but we can tell that there 
ought to be both very divergent fertile forms and slightly divergent 
non-fertile forms, and this is the case. 
It has also been objected that the gradual increase of sterility, 
as we pass from different species to different genera and families, 
proves that divergence produces sterility. But it would exist 
on my theory; for if physiological isolation, more or less 
complete, occurs before each species is formed, it will have 
occurred at least twice between the members of two genera, and 
more often between those of two families. If B is separated 
from A by being nearly infertile, and C from B in the same 
way, C is likely to be still more infertile with A. But in some 
cases geographical or other isolation takes the place of physio- 
logical isolation, and then any number of successive divergences 
may occur without any accompanying infertility. 
It has been said (I have lost the reference) that a certain 
amount of sterility has resulted in some cases from the diver- 
gence produced by artificial selection. It may be so. But 
on my theory, physiological isolation, the spontaneous occur- 
rence of a fertility circumscribed by the boundary of common 
parentage, must be of very common occurrence, since it must 
have occurred not only once for each of most of the recognised 
species, but many more times when the resulting species has 
died out, and in some cases where the two species, though still 
existing, have not diverged in any way so as to suggest to 
observers that they are not one, (just as many island species do 
not differ perceptibly from those on the mainland). If spon- 
taneous physiological isolation is so common, it would be 
certain to occur, at any rate in its commoner partial form, among 
the great variety of our domesticated animals, even if, as I 
believe, ordinary variation has no tendency to produce it. 
EDMUND CATCHPOOL 
Friends’ Institute, E.C., October 13 
Note upon the Habits of Testacella 
BETWEEN four and five months ago I found eleven specimens 
of this slug upon a low wall surrounding the garden of a house 
