NA TURE 



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THURSDAY, MAY 13, 1909. 



THE ORIGIN OF VERTEBRATES. 

 The Origin of Vertebrates. By Dr. Walter Holbrook 

 Gaskcll, F.R.S. Pp. iv+537; 168 figures. 

 (London : Longmans, Green and Co., 1908.) Price 

 21s. net. 



TWENTY years ago the author of this interesting 

 book was led by his studies on the innervation 

 of the heart to make a comparison between the 

 central nervous system in vertebrates and that in 

 appendiculate invertebrates. This led him to a highly 

 original theory of the derivation of vertebrates from 

 an arthropod stock, and the researches of twenty 

 years have strengthened his confidence in this con- 

 clusion. Encouraged by what Hu.xley wrote to him 

 in 1889, " There is nothing so useful in science as 

 one of those earthquake hypotheses, which oblige one 

 to face the possibility that the solidest-looking struc- 

 tures may collapse," Dr. Gaskell has published paper 

 after paper in support of the view that the infundi- 

 bulum may represent the old oesophagus, the ven- 

 tricles of the brain the old cephalic stomach, the 

 canal of the spinal cord the long straight intestine, 

 the cranial segmental nerves the infra-oesophageal 

 ganglia, the cerebral hemispheres and optic and ol- 

 factory nerves the supra-cesophageal ganglia, and the 

 spinal cord the ventral chain of ganglia. 



" Not having been educated in a morphological 

 laboratory and taught that the one organ which is 

 homologous throughout the animal kingdom is the 

 gut, and that therefore the gut of the invertebrate 

 ancestor must continue as the gut of the vertebrate, 

 the conception that the central nervous system has 

 grown round and enclosed the original ancestral gut, 

 and that the vertebrate has formed a new gut, did 

 not seem to me so impossible as to prevent my 

 taking it as a working hypothesis, and seeing to 

 what it would lead." 



As is well known, there are various rival theories 

 as to the origin of vertebrates, though the prevalent 

 position is agnostic. Thus an attempt has been made 

 to derive vertebrates from annelids by supposing a 

 reversal of surfaces, but the author regards the diffi- 

 culties of this hypothesis as "insuperable." On 

 another view the annulate and the vertebrate types 

 had a separate origin ; in the former, the digestive 

 tube pierced the central nervous system and was 

 situated dorsally to its main mass; in the latter, the 

 segmented central nervous system was situated from 

 the first dorsally to the alimentary canal, and was 

 not pierced by it. According to Gaskell, this theory 

 does not explain the tubular appearance of the central 

 nervous system. This, which seems to some an un- 

 important architectural consequence of the mode of 

 development from a medullary groove, is to Gaskell 

 a recapitulation of the way the nerve cord grew 

 round the old gut. Gaskell also says that the extra- 

 ordinary resemblance between the structure and ar- 

 rangement of the central nervous systems of verte- 

 brates and arthropods is against the view of their 

 phyletic distinctness. But, given segmentation in two 

 NO. 2063, VOL. 80I 



distinct types, we naturally expect similarity in the 

 general plan of innervation. 



Dr. Gaskell thinks that the nervous system fur- 

 nishes the most important clues to relationship, and 

 arthropods alone possess a central nervous system 

 closely comparable with that of vertebrates. " The 

 vertebrate tissues resemble more closely those of the 

 arthropod than of any other invertebrate group." 

 Argument from analogy " compels one to the conclu- 

 sion that the fishes arose from the race which was 

 dominant at the time when the fishes first appeared," 

 i.e. from the Palseostraca. And do not the ancient 

 fishes, like Pteraspis, Cephalaspis, and Pterichthys, 

 resemble in a remarkable manner members of the 

 Pala;ostracan group, " so that again and again 

 palaeontologists have found great difificulty in deter- 

 mining whether a fossil is a fish or an arthropod " ? 

 Thus various lines of argument indicate the origin 

 of vertebrates from arthropods, or, more precisely, 

 that the vertebrate was formed from the Palaeostracan 

 without any reversal of surfaces, but by the amal- 

 gamation of the central nervous system and the ali- 

 mentary canal. The vertebrate's cerebral hemispheres 

 and basal ganglia correspond to the supra-cesophageal 

 ganglia of the arthropod, the crura cerebri to the oeso- 

 phageal commissures, the infra-infundibular part of 

 the brain to the sub-oesophageal ganglia, the in- 

 fundibular tube to the oesophagus, the third ventricle 

 to the cephalic stomach, the canal of the spinal 

 cord to the intestine. The vertebi-ate's gut is, of 

 course, a new formation " necessitated by the 

 urgency of the case." Its homology with the inverte- 

 brate gut is a morphological illusion. It is only an 

 analogue. 



All sorts of difficulties rise in- the mind as one 

 considers this hypothesis, but the author is nothing 

 if not ingenious in meeting them. Our old clues — 

 through lancelets, tunicates, and enteropneusts — 

 are brushed aside, and the ammoccete — so peculiar 

 in many ways — is trusted to as the lowest perfect 

 vertebrate. The highly specialised character of 

 Limulus and the Palseostraca would deter many 

 from looking to them as even near probable origina- 

 tors ; but this is not the author's view. If the in- 

 fundibular tube be " oesophagus," the third ventricle 

 " cephalic stomach," the spinal canal " intestine," and 

 the neurenteric canal the old way to the anus, we 

 land in difficulties which seem to us as insuperable 

 as those of the reversal hypothesis seem to the author. 

 We want to know, for instance, where the arthropod's 

 mesenteron has gone. But this is only one of the 

 most obvious difficulties, and it is no difficulty to the 

 author, who throws the germ-layer theory overboard 

 as a morphological anachronism, a survival of a 

 dogma due to the lively imagination of Haeckel. 

 ' In his second chapter Dr. Gaskell finds support 

 for his thesis in the eyes. The pineal gland repre- 

 sents a pair of median eyes ; Ostracoderms had 

 median as well as lateral eyes ; so has the king-crab, 

 and so had Eurypterids. The inverted retinas of the 

 vertebrate lateral eyes find their counterpart in the 

 lateral eyes of arachnids, and the Palseostraca were 

 ancestral to both. But do not the vertebrate lateral 



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