5^4 



NA TURE 



[OCTOIIF.K 8, 1S96 



The first proposition is a well-known embryological fact. The 

 second proposition is also well known for all vertebrates, and is 

 especially well exemplified in the embryological development of 

 Ammoccetes, according to Shipley. The third proposition is 

 also well known, and has received valuable enlargement in the 

 recent researches of Langley and Anderson (Journ. of P/iysiohiy, 

 vols, xviii., xix.). 



Further, we see that in this part of the body the ancestor 

 of the vertebrate must have had a ccclomic cavity the walls 

 of which were innervated, not from the mesosomatic nerves 

 or respiratory nerves, but from the metasomatic group of nerves ; 

 and in connection with this body cavity there must have 

 existed a kidney apparatus, also innervated by the metasomatic 

 nerves ; with the repetition of segments by which the elongation 

 of the animal was brought about the body cavity was elongated, 

 and the kidney increased by the repetition of similar excretory 

 organs. All, then, that is required in the original ancestor in 

 order to obtain the permanent body cavity and urinary organs 

 characteristic of the vertebrate is to postulate the presence of a 

 permanent body cavity in connection with a single pair of urinary 

 tubes in the metasomatic region of the body. As yet I have not 

 worked out this part of my theory, and am therefore strongly 

 disinclined to make any assertions on the subject. I should like, 

 however, to point out that, according to Kishinouye (Journ. of 

 Coll. of Sii. Tokio, vol. iv. 1S90, vol. vi. 1S94), a permanent 

 body cavity does exist in this part of the body in spiders, known 

 by the name of the .stercoral pocket ; into this ccelomic cavity 

 the excretory Malphigian tubes open. 



The PaliEontological Evidence. 



It is clear, from what has already been said, that the pala;onto- 

 logical evidence ought to show, first, that the vertebrates 

 appeared when the waters of the ocean were peopled with the 

 forefathers of the Crustacea and Arachnida, and, secondly, the 

 earliest fish-like forms ought to be characterised by the presence 

 of a large cephalic part to which is attached an insignificant body 

 and tail. 



Such was manifestly the case, for the earliest fish-like forms 

 appear in the midst of and succeed to the great era of strange 

 proto-crustacean animals, when the sea swarmed with Trilobites, 

 Eurypterus, SUmonia, Limulus, Pterygotus, Ceratiocaris, and 

 a number of other semi-crustacean, semi-arachnid creatures. 

 When we examine these ancient fishes we find such forms as 

 Pteraspis, Pterichthys, Astrolepis, Bothriolepis, Cephalaspis, all 

 characterised by the enormous disproportion between the extent 

 of the head region and that of the body. Such forms would 

 have but small power of locomotion, and further evolution con- 

 sisted in gaining greater rapidity and freedom of movements by 

 the elongation of the abdominal and tail regions, with the result 

 that the head region became less and less prominent, until finally 

 the ordinary fish-like form was evolved, in which the head and 

 gills represent the original head and branchial chamber, and the 

 flexible body, with its lateral line nerve and intestine innervated 

 by the vagus nerve, represents the original small tail-like body 

 of such a form as Pterichthys. 



Nay, more, the very form of Pterichthys and the nature of its 

 two large oar-like appendages, which, according to Traquair, are 

 hollow, like the legs of insects, suggest a form like Eurypterus, 

 in which the remaining locomotor appendages had shrunk to 

 tentacles, as in Ammoccetes, while the Urge oar-like appendages 

 still remained, coming out between the upper and lower lips and 

 assisting locomotion. The Ammocoetes-like forms which in all 

 probability existed between the time of Eurypterus and the time 

 of Pterichthys have not yet been found, owing possibly to the 

 absence of chitin and of bone in these transition forms, unless we 

 may count among them the recent find by Traquair of Palaeo- 

 spondylus Gunni. 



The evidence of palaeontology, as far as it goes, confirms 

 absolutely the evidence of anatomy, physiology, phylogeny, and 

 embryology, and assists in forming a perfectly consistent and 

 harmonious account of the origin of vertebrates, the whole 

 evidence showing how nature made a great mistake, how 

 excellently she rectified it, and thereby formed the new and 

 mighty kingdon of the Verlebrata. 



ConsiiUration of Rival Theories. 



In conclusion I would ask. What are the alternative theories of 



the origin of vertebrates? It is a strange and striking fact how 



often, when a comparative anatomist studies a particular 



invertebrate group, he is sure to find the vertebrate at the end 



of it : it matters not whether it is the Nemertines, the 

 Capitellids, Balanoglossus, the Helminths, Annelids, or 

 Echinoderms ; the ancestor of the vertebrate is bound to be in 

 that particular group. Verily I believe the Mollusca alone have 

 not yet found a champion. On the whole I imagine that two 

 views are most prominent at the present day: (l) to derive 

 vertebrates from a group of animals in which the alimentary 

 canal has always been ventral to the nervous system ; and (2) to 

 derive vertebrates from the appendiculate group of animals, 

 especially annelids, by the supposition that the dorsal gut of the 

 latter has become the ventral gut of the former by reversion ot 

 surfaces. Upon this latter theory, whether it is Dohrn or van 

 Beneden or Patten who attempts to homologise similar parts, it 

 is highly amusing to see the hopeless confusion into which they 

 one and all get, and the extraordinary hypijtheses put forward 

 to explain the fact that the gut no longer pierces the brain. One 

 favourite method is to cut off the most important part of the 

 animal, viz. his supra.ce.sophageal ganglia, then let the mouth 

 open at the anterior end of the body, turn the animal over, so 

 that the gut is now ventral, and let a new brain, with new eyes, 

 new olfactory organs, grow forward from the infra-cesophageal 

 ganglia. Another ingenious method is to separate the two 

 supra-resophageal ganglia, let the mouth tube sling round 

 through the separated ganglia from ventral to dorsal side, then 

 join up the ganglia and reverse the animal. The old attempts 

 of Owen anci Dohrn to pierce the dor.sal part of the brain with 

 the gut tube either in the region of the pineal eye or of the 

 fourth ventricle have been given up as hopeless. Still the 

 annelid theory, with its reversal of surfaces, lingers on, even 

 though the fact of the median pineal eye is sufficient alone to 

 show its absolute worthlessness. 



Then, as to the other view, what a demand does that make 

 upon our credulity ! We are to supjjose that a whole series of 

 animals has existed on the earth, the development of which has 

 run parallel with that of the great group of appendiculate animals, 

 but throughout the group the nervous system has always been 

 dorsal to the alimentary canal. Of this great group no trace 

 remains, either alive at the present day or in the record of the 

 rocks, except one or two aberrant, doubtful forms, and the group 

 of Tunicates and Amphioxus, both of which are to be looked 

 upon as degenerate vertebrates, and indeed are more nearly 

 allied to the Ammoccetes than to any other animal. This hypo- 

 thetical group does not attempt to explain any of the peculiarities 

 of the central nervous system of vertebrates ; its advocates, in 

 the words of Lankester, regard the tubular condition of the 

 central nervous system as in its origin a purely developmental 

 feature, possessing no phylogenetic importance. Strange jxiwer 

 of mimicry in nature, that a tube so formed should mimic in iti> 

 terminations, in its swellings, in the whole of its topographical 

 relations to the nervous masses surrounding it the alimentary 

 canal of the other great group of segmented animals so closely 

 as to enable me to put before you so large a number ot 

 coincidences. 



Just imagine to yourselves what we are required to believe ! 

 We are to suppose that two groups of animals have diverged 

 from a common stock somewhere in the region of the 

 Coelenterata, that each group has become segmented and 

 elongated, but that throughout their evolution the one group has 

 possessed a ventral mouth, with a ventral nervous system and a 

 dorsal gut, while in the other — the hypothetical group — the 

 mouth and gut have throughout been ventral and the nervous 

 system dorsal. Then we are further to sujipose that, without 

 being able to trace the steps of the process, the central nervous 

 system in the final members of this hypothetical group has taken 

 on a tubular form of so striking a character that every part ot 

 this dorsal nerve-tube can be compared to the dorsal alimentary 

 tube of ihe other great group of appendiculate animals. The plain, 

 straightforward interpretation of the facts is what I have put 

 before you, and those who oppose this interpretation and hold 

 to the inviolability of the alimentary canal are, it seems to me, 

 bound to give a satisfactory explanation of the vertebrate nervous 

 .system and pineal eye. The time is coming, and indeed has 

 come, when the fetish-worship of the hy]ioblast will give way to 

 the acknowledgment that the soul of every individual is to be 

 found in the brain, and not in the stomach, and that the true 

 principle of evolution, without which no upward progress is 

 possible, consists in the steady upward development of the 

 central nervous system. 



In conclusion I would like to quote to you a part of the last 

 letter I ever received from Prof Huxley, in which, with reference 



NO. 1406, VOL. 54] 



