5o8 



NA TURE 



[September 20, 1900 



the head, the rays of the dorsal and anal fins are fewer and 

 correspond in their number to that of the internal supports or 

 "interspinous" bones, while in the caudal we see again the 

 semi-heterocercal or abbreviate- heterocercal condition we 

 noticed above in Catopterus. 



Then if we refer to the tail of Lepidosteus itself we shall 

 observe how few are its rays and how evident it is that we have 

 here to do only with the lower lobe of the original palseoniscoid 

 caudal fin. For a convincing corroboration of this we have only 

 to look at the tail of the embryo Lepidosteus as described and 

 figured by Prof. A. Agassiz to see that it in reality passed 

 through an Acipenseroid stage, and the last we see of the upper 

 lobe of this tail is in the form of a filament which projects from 

 the top of the original lower lobe and then disappears. 



Again, in these Lepidosteid forms we have a repetition of the 

 same tendency for the thick rhombic, peg-and-socket articula- 

 ting scales to become rounded and imbricating as we saw in the 

 Crossopterygii and again in the PaloeoniscidEe. So, for instance, 

 in Caturus, which has been shown by Dr. Smith Woodward to 

 resemble Eugnathus so closely in structure, the scales are deeply 

 overlapping, and most of them " cycloidal " in shape. To such 

 an extent does this go that in the recent Amia, whose skeletal 

 structure so clearly shows it to belong to this group, the rounded 

 scales are so thin and flexible that after it was removed from the 

 Clupeoid family, or Herrings, and placed among the " Ganoids" 

 it was considered to be the type of a distinct sub-order of 

 " Amioidei." Ten years ago, however, Dr. Beard came to the 

 conclusion, from anatomical and embryological data, that this 

 division could no longer be maintained, and that the Amioids 

 must in fact be united with the Lepidosteids.^ Dr. Smith 

 Woodward has, therefore, in the third volume of his catalogue, 

 done well to reduce the "Amioidei" to the rank of a family, 

 including also the Jurassic genera Liodesmus and Megalurus, 

 and to place this family close to the Eugnathidse. 



As the Asipenseroids dwindled away after the close of the 

 great Palaeozoic era, and are now scantily represented only by 

 the degenerate paddle-fishes and sturgeons, so the Lepidosteid 

 series, flourishing greatly in the Trias and Jura, in their turn 

 declined in the Cretaceous, and in the Tertiary period became 

 about as much a thing of the past as they are now, the North 

 American Lepidosteus and Amia, of which remains of extinct 

 species have also been found in Eocene and Miocene rocks, only 

 remaining. These two genera can, however, hardly be called 

 "degenerate." 



But that the fishes which succeeded the Lepidosteids in 

 populating the seas and rivers of the globe were evolved from 

 them there can be no reasonable doubt, while it is equally clear 

 that they branched off at an early period, as already in the 

 Trias we find the first representatives of the order of Isospondyli, 

 which contains our familiar Herrings, Salmonids, Elopids, 

 Scopelids, &c. For Dr. Smith Woodward has not only 

 definitely placed the Jurassic Leptolepidae and Oligopleuridse in 

 the Isospondyli, but also the Pholidophoridse, which appear in 

 the Trias and extend to the Purbeck. And it is of special 

 interest that in the Pholidophori the scales are still brilliantly 

 ganoid and mostly retain the peg-and-socket articulation, while 

 in the allied Leptolepidse, although they have become thin and 

 circular, a layer of ganoine mostly remains. 



With the Isospondyli we now get fairly among the bony fishes 

 of modern type — Teleostei as we used to call them — to which 

 other sub-orders are added in Cretaceous and Tertiary times, 

 and which in the present day have assumed an overwhelming 

 numerical preponderance over all other fishes. The prevalent 

 form of scale among these is thin, rounded, deeply imbricating, 

 and with the posterior margin either plain (cycloid) or serrated 

 (ctenoid). But that these " cycloid " and " ctenoid " scales are 

 modifications from the rhombic osseous "ganoid" type we 

 cannot doubt after what we have seen. It is indeed strange 

 that the same tendency to the change of rhotnbic into circular 

 overlapping scales should have occurred independently in more 

 than one group. 



For reasons given at the beginning, and also because I fear I 

 have already exceeded the limit of time usually allotted to such 

 an Address, I must now stop. 



Butinconclusionlmayalludetoa well-known fact regarding the 

 tail of these modern fishes, the bearing of which on the doctrine 

 of Descent is sufficiently clear and has long been recognised. 



1 "The Inter-relationships o{x^^\c\i\^yo^^\As." AnatovtischerAnzeiger, 

 1890. Smith Woodward arrived at the same result in 1893 from the study 

 of the Jurassic genera / cpidoius and Dapedi'is. See Proc. Zool. Soc. Land., 

 June 20, 1893, pp. 559-565- 



NO. 161 2, VOL. 62] 



We have seen that the completely heterocercal tail of the 

 typical Acipenseroid becomes, by abortion of the upper lobe 

 and shortening of the axis, the semi-heterocercal one of the 

 Lepidosteids, in most of which, however, the want of symmetry 

 is still perceptible externally by a short projection or "sinus" 

 of scales which is directed obliquely upward at the beginning of 

 the top of the fin. In the ordinary bony fishes and in some 

 Lepidosteids also the caudal fin becomes likewise symmetrical, 

 as seen from the outside ; generally also bilobate, though the 

 upper lobe is not that of a Palaeoniscid or Sturgeon. This con- 

 dition of tail has been long known as " homocercal." But in 

 many such homocercal tails, when we dissect away the skin and 

 soft parts, the upward bend of the vertebral axis is revealed, and 

 in some, as in the Salmon, the extremity of the vertebral axis 

 is continued as a cartilaginous style among the rays near the 

 upper margin of the fin. But there are many others, such, for 

 instance, as the peculiarly specialised group of Pleuronectidse or 

 flat fishes, in which the skeleton of the caudal extremity looks 

 quite symmetrical, but yet in the embryo the extremity of the 

 notochord is seen to have an upward bend, showing that the 

 homocercal tail is indeed a specialisation on the old heterocercal 

 one. It is strange that though this embryological fact was long 

 ago pointed out by Agassiz, and though he noted its great 

 interest in connection with the prevalence of heterocercy among 

 the Palaeozoic fishes, yet he remained to the end an opponent of 

 evolution. But this is just one of these instances in which 

 Phylogeny and Ontogeny mutually illustrate each other. Why, 

 otherwise, should the tail of the embryo stickleback or flounder 

 be heterocercal ? 



Incompletely as I have treated the subject, it cannot but be 

 acknowledged that the palaeontology of fishes is not less emphatic 

 in the support of Descent than that of any other division of the 

 animal kingdom. But in former days the evidence of fossil 

 ichthyology was by some read otherwise. 



It is now a little over forty years since Hugh Miller died : he 

 who was one of the first collectors of the fossil fishes of the 

 Scottish Old Red Sandstone, and who knew these in some 

 respects better than any man of his time, not excepting Agassiz 

 himself. Yet his life was spent in a fierce denunciation of the 

 doctrine of evolution, then only in its Lamarckian form, as 

 Darwin had not yet electrified the world with his " Origin of 

 Species." Many a time I wonder greatly what Hugh Miller 

 would have thought had he lived a few years longer, so as to 

 have been able to see the remarkable revolution which was 

 wrought by the publication of that book. 



The main argument on which Miller rested was the " high " 

 state of organisation of the ancient fishes of the Palaeozoic 

 formations, and this was apparently combined with a confident 

 assumption of the completeness of the geological record. As to 

 the first idea, we know of course that evolution means the 

 passage from the more general to the more special, and that 

 although as the general result an onward advance has taken 

 place, yet specialisation does not always or necessarily mean 

 " highness" of organisation in the sense in which the term is 

 usually employed. As to the idea of the perfection of the geo- 

 logical record, that of course is absurd. 



We do not and cannot know the oldest fishes, as they would 

 not have had hard parts for preservation, but we may hope to 

 come to know many more old ones, and older ones still, than we 

 do at present. My experience of the subject of fosil ichthyology 

 is that it is not likely to become exhausted in our day. 



We are introduced at a period far back in geological history 

 to certain groups of fishes some of which certainly are high in 

 organisation as animals, but yet of generalised type, being fishes 

 and yet having the potentiality of higher forms. But, because 

 their ancestors are unknown to us, that is no evidence that they 

 did not exist, and cannot overthrow the morphological testiinony 

 in favour of evolution with which the record actually does furnish 

 us. We may therefore feel very sure that fishes, or " fish-like 

 vertebrates," lived long ages before the oldest forms with which 

 we are acquainted came into existence. 



The modern type of bony fish, though not so "high" in many 

 anatomical points as that of theSelachii, Crossopterygii, Dipnoi, 

 Acipenseroidei and Lepidosteoidei of the Palaeozoic and 

 Mesozoic eras, is more specialised in the direction of the fish 

 proper, and, as already indicated, specialisation and " highness " 

 in the ordinary sense of the word are not necessarily coincident. 

 But ideas about these things have undergone a wonderful change 

 since those pre-Darwinian days, and though we shall never be able 

 fully to unravel the problems concerning the descent of animals, we 

 tee many things a great deal more clearly now than we did then. 



