528 
The notion, which until recently prevailed, that among these 
gastropods the non-twisted or so-called euthyneurous condition 
of the visceral nerve-cords, as exemplified by the Opistho- 
branchs, is a direct derivative of that of the Chitons has been 
proved to be erroneous, since the nerves in Actzeon and Chilina, 
like those of the prosobranchs, are twisted or streptoneurous. 
And as to the torsion of the gastropod body, recent research, 
in which one of my pupils has played a part, involving the dis- 
covery of paired reno-pericardial apertures in Haliotis, Patella 
and Trochus, has resulted in proof that the dextral torsion 
which leads to the monotocardiac condition does not uniformly 
affect all organs lying primitively to the lefi of the rectum, as 
we have been taught ; since, concerning the renal organs, it is 
the pr zmztively (pretorsional) left one which remains as the 
functional kidney, its ostium as the genital aperture. Nor is 
the primitively right kidney necessarily lost, for while its ostium 
remains as the renal orifice, its body, by modification and re- 
duction, may become an appendage of the functional kidney, 
the so-called nephridial gland. And we now know there are 
cases of sinistral torsion of the visceral hump, in which the 
order of suppression of the organs is not reversed, the arrange- 
ment being one of adaptation of a dextral organisation to a 
sinistral shell. 
Though thus specialised and asymmetrical as a group, the 
gastropods are yet plastic to an unexpected degree. Madagascar 
has yielded a Physa (P. /ame//ata) with a neomorphic gill, a 
character shared by species of Planorbis (P. cormeus and P. mar- 
gimatus), and an Ancylus in which the lung-sac is suppressed ; 
while St. Thomas's Island has given us a snail (7hyrophorella 
Thomensts), the peristome of whose shell is produced into a 
protective lid. 
In palzontology, history records the fact that in 1864 Huxley 
observed that the genus Belemnites appears to have borne but 
six free arms ; a startling discovery which Jay dormant till the 
present year, And the recent study of the fauna of the great 
African lakes, in bringing to light the existence of a halolimnic 
molluscan series in Lake Tanganyika, has opened up new pos- 
sibilities concerning the paleontological resources of enormous 
aqueous deposits, recently discovered in the interior, and has 
entirely changed our geological conceptions of the nature of 
Equatorial Africa. 
Time prevents my dealing with other groups, and it must 
suffice to say that with those I have not considered substantial 
work has been done. From what has been said, it is natural to 
expect that in some direction or another so vast an accumulation 
of facts must have extended the Darwinian teaching ; and it 
is now quite clear that this has been the case with the two 
post-Darwinian principles known as ‘‘ Substitution” and 
Isomorphism or ‘‘ Convergence.” 
The former may be exemplified by nothing better than the 
case of the Rays and Skates, in which, under the usurpation of 
the propelling function of the tail by the expanded pectoral fins, 
the tail, free to modify, becomes in one species a lengthy whip- 
lash, in another a vestigial stump, in others, by the development 
of powerful spines, a formidable organ of defence. In both the 
Rays and certain other fishes subject to the working of this law, 
modification goes further still, in the appearance of electric 
organs in remotely related genera and species, by specialisation 
of the muscular system of the trunk or tail, or, as in the case of 
Malapterurus, of ‘‘ tegumental glands.” In this we have a diffi- 
culty admitted by Darwin himself, which now becomes clear 
and intelligible, since there is nothing new. There has simply 
come about the conversion, in one case of the energy of mus- 
cular contraction, in the other of glandular secretion, into that 
of electrical discharge, with accompanying structural change. 
The blind locust (Pachyramina fuscifer) of the New Zealand 
Limestone caves presents an allied case, since here, under the 
reduction of the eye, the antennz, elongated to a remarkable 
degree, have become the more efficiently tactile; and it is an 
interesting question whether this principle may not explain the 
attenuation of the limbs in the recently discovered American 
Proteoid (Zyphlomolga Rathburnz) of the Texan subterranean 
waters. 
And as to isomorphism, by which we mean the assumption 
of a similar structural state by members of diverse or indepen- 
dent groups, I would recall the case of the Eocene Creodont 
Patriofelis and the Seals, and that of the Myriapods to which I 
have already alluded, and would cite that of the Dinosaurs 
and Birds, heterodox though it may appear, for reasons I have 
given. 
NO. 1717, VOL. 66] 
NATURE 
| 
[SEPTEMBER 25, 1902 
As our knowledge increases, there is every reason to believe 
that, in the non-appreciation of these principles in the past, not 
a few of our classifications are wrong. We have even had our 
bogies, as, for example, the so-called Physemaria, which de- 
ceived the very elect; and before I close I wish to deal briefly 
with a question of serious deubt, which these considerations 
suggest. ‘ 
It is that of the position in the zoological series of the Limu- 
loids, popularly termed the King Crabs. These creatures, best 
known from the opposite shores of the Northern Pacific, but 
found in the oriental seas as well as far south as Torres Strait, 
have been since 1829 the subject of a difference of opinion as to 
their zoological position and affinities. Within the last twenty 
years there have been three determined advances upon them, 
and of these the third and most recent may be first discussed. 
It has for its object the attempt to prove that they are intimately 
associated with the cephalaspidian and other shield-bearing 
fishes of the Devonian and Silurian epochs, and that through 
them they are ancestral to the Vertebrata. The latest phase of 
this idea is based on the supposed existence in a Cephalaspis of 
a series of twenty-five to thirty lateral appendages of arthropod 
type. When, however, it is found that the would-be limbs are 
but the edges of body-scutes misinterpreted, suspicion is aroused ; 
and when, working back from this, an earlier altempt reveals 
the fact that the author, compelled to find trabeculz, in order to 
force a presupposed comparison between the architecture of the 
Cephalaspidian head-shield and the Limulus’ prosomal hood, 
resorts to a comparison between the structure of the former in 
general and that of the cornu of the latter, with details which 
on the piscine side are not to date, the argument must be 
condemned, It violates the first principles of comparative 
morphology, and is revolting to common sense; and as to the 
fishes concerned, we know that they have nothing whatever to 
do with the Limuloids, for we have already seen that, with 
their allies the Pteraspidiee, they are a lateral branch of the 
ancestral piscine stem. ‘ 
The second advance upon the king crabs has very much in 
common with the first. It has engrossed the attention of an 
eminent physiologist for the last six or seven years, and by him 
it was in detail set before Section I at our meeting of 1896. 
Suffice it to say that it specially aims at establishing a structural 
community between the king crabs and certain vertebrates, 
favourable to the conviction that the Vertebrata have had an 
arthropod ancestry. When we critically survey the appalling 
accumulation of words begotten of this task, it is sufficient to 
consider its opening and closing phases. At the outset, under 
the conclusion that the vertebrate nervous axis is the metamor- 
phosed alimentary canal of the arthropod ancestor, the necessity 
for finding a digestive gland is mainly met by homologising the 
so-called liver of the arthropod with the cellular arachnoid of 
the larval lamprey, in violation of the first principles of com- 
parative histology! At the close we find ingenious attempts to 
homologise nerve tracts and commissures related to the organs 
of sense, such as are invariably present wherever such organs 
occur. Sufficient this to show that the comparison, in respect 
to its leading features, is in the opening case strained to an 
unnatural degree, in the closing case no comparison at all. 
Finding, as we do, that the rest of the work is on a par with 
this, we are compelled to reject the main conclusion as un- 
natural and unsound ; and when we seek the explanation of 
this remarkable course of action, we are forced to believe that 
it lies in the failure to understand the nature of the morpho- 
logical method. For the proper pursuit of comparative mor- 
phology, it is not sufficient that any two organisms chosen here 
and there should be compared, with total disregard of even 
elementary principles. Comparison should be first close and 
with nearly related forms, passing later into larger groups, with 
the progressive elimination of those characters which are found 
to be least constant. And necessary is it, above all things, that 
in instituting comparison it should be first ascertained what it is 
that constitutes a crustacean a crustacean, a marsipobranch a 
cyclostome, and so on forthe rest. We have tried to accept this 
theory, fascinated both by the arguments employed and by the 
idea itself, which for ingenuity it would be difficult to beat, but 
we cannot; and we dismiss it as misleading, as a fallacy, 
begotten of a misconception of the nature of the morphological 
method of research. It is of the order of events which led 
Owen to compare a cephalopod and a vertebrate, led Lacaze- 
Duthiers to regard the Tunicata and Lamellibranchs as allied ; 
and with these and other heresies it must be denounced. 
