TRANSACTIONS OF SECTION D. 631 



tody, by modification and reduction, may become an appendage of the functional 

 iidney, 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 arrangement 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. lamellata) 

 with a neomorphic gill, a character shared by species of Flmiorhis (P. corneus and 

 P. marginatus), and an Ancyhis in which the lung-sac is suppressed ; '^^ while 

 St. Thomas's Island has given xis a snail (Thyropkorella thomensis), the peristome 

 of whose shell is produced into a protective lid.''*" 



In palfeontology, 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 lay 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 moUuscan 

 series in Lake Tanganyika, has opened up new possibilities concerning the palseon- 

 tological resources of enormoiis 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 state 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 fimction of the tail by 

 the expanded pectoral fins, the tail, free to modify, becomes in one species a 

 lengthy whiplash, 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 difficulty admitted by 

 Darwin himself, which now becomes clear and intelligible, since tliere is nothing 

 new. There has simply come about the conversion, in one case of the energy of 

 muscular contraction, in the other of glandular secretion, into that of electrical 

 discharge, with accompanying structural change. The blind locust {Fachyramina 

 fuscifer) of the New Zealand Limestone caves presents an allied case, since here, 

 under the reduction of the eye, the antennae, 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 dis- 

 covered American Proteoid {Tyjihlomolga rathburni) of the Texan subterranean 

 waters."^ 



And as to isomorphism, by which wo mean the assumption of a similar 

 structural state by members of diverse or independent groups, I would recall the 

 case of the Eocene Greodort 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. 



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 deceived the very elect ; '''^' and before I close I wish to deal brieily with a 

 question of serious doubt, which these considerations suggest. 



It is that of the position in the zoological series of the Limuloids, 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 



