360 
NATURE 
[ August 19, 1886 
800 pages, and when we consider the position which Prof. 
Kopp holds as an historian of chemistry, a position which 
demands that due regard should be paid to his writings. 
The more immediate object of the pamphlet is, how- 
ever, to reply to the strictures of MM. Bartoli and 
Stracciati on the law of molecular volumes. These gentle- 
men have criticised somewhat severely Kopp’s work in this 
department without taking into consideration the obstacles 
which, at the time it was carried out (1855) stood in the 
way of accurate and definite investigation. Singularly 
enough, they do not escape the same charge, having them- 
selves in some cases made use of materials of an insufti- 
cient degree of purity. It is likewise pointed out that 
they are labouring under a complete misapprehension of 
the views held by Kopp and the significance of his deduc- 
tions, nor do they seem to have appreciated the difficulties 
that surround the establishment of a “ physical law” of 
general application. GAH. B. 
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 is so great 
that it is impossible otherwise to insure the appearance even 
of communications containing interesting and novel facts.] 
Organic Evolution 
ALLUDING to the first instalment of your abstract of my 
recently published paper on ‘‘ Physiological Selection,” the 
Duke of Argyll remarks :— 
“*T rejoice that the author has at last discovered that ‘ natural 
selection has been made to pose as a theory of the origin of 
species, whereas in point of fact it is nothing of the kind.’ 
This has been my contention for many years” (NATURE, vol. 
XXXIV. Pp. 335). 
These words seem to imply that my views with regard to 
natural selection have now undergone an important change, and 
one which brings them into conformity with those that have for 
many years been contended for by the Duke. It therefore seems 
desirable to state that such isnot the case; and as [ can only 
attribute the misunderstanding of so able and friendly a reader to 
some ambiguity in the condensed abstract of my paper to which 
he refers, I may invite him to consult the paper itself, where the 
matter to which he alludes is more fully explained. He will 
there find that my views upon the subject of natural selection are 
the same now as they have been during the last fifteen years ; 
that in all essential respects they still coincide with those that 
were held by Mr, Darwin ; and that my ‘‘ additional suggestion 
on the origin of species, although quite independent of natural 
selection, is in no way opposed to natural selection,” but is to 
be regarded as indicating ‘‘a factor supplementary to natural 
“selection.” 
The state of matters, then, is simply this. Mr. Darwin him- 
self has freely acknowledged that his theory of natural selection 
is not in itself a sufficient explanation of the origin of species. 
He therefore supplemented the natural causes which are together 
comprised under this term? by sundry other causes of a similarly 
1 In common with many other critics of Mr. Darwin’s work, the Duke of 
Argyll has always contended that the theory of natural selection is “ funda- 
mentally erroneous” in that it assumes “ variations to arise by accident,” and 
merely “groups under one form of words, highly charged with metaphor, an 
immense variety ofcauses.some purely mental, some purely vital, and others 
purely physical or mechanical.’’ This, however, is no valid criticism of the 
theory, which for the first time did comprise under one general point of view all 
the causes which together go to produce the results. In the opinion of the 
Duke, the weakest element of the theory consists in its inability to explain 
the causes of those variations on the occurrence of which the theory depends 
(Narure, loc ci#.. p. 336). But it is clearly no valid objection to a theory 
which explains one set of causes that it is unable to explain another and 
ulterior set. So long as variations of all kinds are known to be matters of 
fact, they are available for the theory of natural selection, even though the 
ulterior or physiological causes of variation should never be discovered. 
And for all the purposes of this theory it makes no difference whether the 
variations which are seen to take place in all directions, with and without 
respect to utility, are spoken of as ‘‘accidental” or as due to hidden causes. 
All that this theory has to do is to take the principle of promiscuous varia- 
tion in all directions as a datum supplied by observation, and from this fact 
to show how the further principles of heredity, struggle, and survival are 
natural kind. Of these he attributed most importance to use, 
disuse, sexual selection, correlated variation, and prolonged 
exposure to uniform conditions of life. To these supplementary 
causes Moritz Wagner added independent variation in the ab- 
sence of intercrossing with parent forms, while I have myself 
added physiological selection. Now, the whole body of Mr. 
Darwin’s followers have agreed with him in holding that the 
theory of natural selection is not in itself a sufficient explanation 
of the origin of species. But many of these followers differ 
from Mr, Darwin, and also differ among themselves, as to the 
proportional part which the principle of natural selection is to 
be considered as having played in the evolution of species. Mr. 
Darwin thought that in this respect natural selection plays a more 
important part than any other principle [therefore it is hard 
to see how in this respect any of ‘‘the successors of Darwin” 
can possibly ‘‘have run quite wild from the teaching of their 
master’’], while in the opinion of many of his followers this 
principle should be regarded as of a value subordinate to the 
others. Of all the writers who have taken the latter view, the 
most clear-headed, as well as the earliest and most persistent, is 
Mr. Herbert Spencer. He more than any ‘other -author has 
been instant, both in season and out of season, in giving reasons 
for the scepticism that is in him. I confess, therefore, to not 
understanding the Duke of Argyll when he says that in the two 
articles recently published by Mr. Spencer ‘‘we have for the 
first time an avowed and definite declaration against some of 
the leading ideas on which the Mechanical Philosophy depends.” 
So far as I can see, these two articles convey little more than 
a reiteration of the characteristically Spencerian view that, in 
the course of organic evolution the processes of “ direct equili- 
bration ” have been of more importance than those of ‘‘ indirect 
equilibration.” By the first of these terms Mr. Spencer means 
use, disuse, and all other causes tending directly to the produe- 
tion of adaptive structures, while by the second he means natural 
selection. Now, from the time when Mr. Darwin first published 
his “‘ Origin of Species” the main point of difference between 
his views and those of Mr. Spencer has uniformly consisted 
in the estimates which they have formed of the relative 
importance of these two kinds of equilibration. Surely, 
therefore, this cannot be the respect in which it is said 
that Mr. Spencer has ‘‘now for the first time opened his 
eyes and his mouth.” Yet, if not, I do not understand the 
allusion. The Duke seems to imagine that, in some way or 
another, Mr. Spencer has taken a new and important ‘ point of 
departure” in the course of his speculative thinking, and one 
which is ‘‘fatal to the adequacy of the Mechanical Philosophy 
as any explanation of organic evolution.” It would be a matter 
of great interest to me —and I am sure to many others who have 
read the articles in question—to be told in what respect Mr. 
Spencer has committed himself to so great a change of doctrine ; 
and it would certainly be a matter of profound astonishment to 
all evolutionists if Mr. Spencer can be shown to have so much 
as insinuated that his ‘direct equilibration” differs from Mr. 
Darwin’s natural selection in not belonging to the system of so- 
called Mechanical Philosophy—or that those factors of organic 
evolution on which he has mainly relied differ from those on 
which Mr. Darwin has mainly relied in lending better counten- 
ance to the Supernatural Philosophy of Design. 
My own attitude with regard to all these questions is perfectly 
plain and simple. In common with Darwin, Spencer, and the 
great majority of evolutionists, I believe that in the origin and 
development of adaptations—whether structural or instinctive— 
two sets of strictly natural causes have been at work: I agree 
with Mr. Darwin in thinking that of these two sets of causes the 
“indirectly equilibrating ” have been of more importance than 
the ‘directly equilibrating”: but I differ from other evolu- 
tionists, both of the Darwinian and Spencerian schools, in 
expressly drawing a marked distinction between causes of either 
kind which have been operative in the evolution of adaptations 
and those which have been operative in the evolution of sfecies ; 
and, lastly, I claim to have shown that when once this distinction 
is recognised, the theory of natural selection ceases to be, 
properly speaking, a theory of the origin of species; that it is 
thus liberated from all the difficulties with which it has hitherto 
been entangled on account of its having been made to ‘ pose” 
as such ; and that it is therefore placed in a position of greater 
competent to se/ect the variations which happen to be useful from those 
which are not. One might as well object to the physical explanation of 
specific gravity in selecting or sorting the different materials of a sea-shore, 
on the ground that we do not know the causes either of gravity in general 
or of the variations that are observable among specific gravities in particular. — 
