Walter Holbrook Gashell. 



xxxm 



" This is not the place to discuss the strong and weak points of his hypothesis 

 that vertebrates are descended from some Crustacean-like ancestor, i.e. from 

 some vaguely reconstructable stock of which the palaeozoic Trilobites, King, 

 crabs and Scorpions are the only known representatives on the invertebrate 

 side, and he bridged the gulf between them and the vertebrates by the 

 Silurian Ostracoderms, of whose internal organisation the larvte of the 

 Lampreys, before their marvellous changes into the present adult forms, seemed 

 to afford a clue. The gulf was great indeed, but his planned bridges were 

 not more hazily sketched than those which pretend to connect the vertebrates 

 either separately or conjointly with Amphioxus, Tunicates, Balanoglossus, etc., 

 with worms and even with Echinoderms. Especially the various worm- 

 theories he considered as no solution of the problem, since they would carry 

 the connection so far back as to merge almost into the beginning of the 

 Metazoa, amounting to no recognisable origin. He on the contrary believed 

 that ' each higher group of animals has arisen in succession from the highest 

 race developed up to that time.' 



" Further, as the leading motif of the whole course of this solution he 

 discerned the orderly sequence in the development of the central nervous 

 system, in which no break of continuity can possibly have occurred. The 

 brain and nerves afford the fundamental homologies ; the organs which they 

 innervate may fall into line in a surprising way, but they are not the 

 essential comparisons, e.g. a new gut may be formed, as in the transforming 

 Ammocoetes. ' The secret of evolutionary success is the development of a 

 superior brain.' 



"The immediate starting point of Gaskell's investigations on the origin of 

 vertebrates was the recognition of the close similarity in structure and 

 function of the different parts of the vertebrate brain with those of Arthropods. 

 The segmental character of the vertebrate central nervous system, so clear to 

 the physiologist, and long before insisted upon by most anatomists, had lost 

 weight for the morphologists, clearly because the C.N.S. appears embryonically 

 as a single unsegmented tube. Here then was the next question forced upon 

 Gaskell's attention. Cannot the two opposing views be reconciled by the 

 suggestion that the vertebrate C.N.S. consists of two parts, closely entangled, 

 viz., a segmental nervous system on the same plan as that of the Arthropods, 

 which is outside and has surrounded an epithelial tubular structure ? 



" This idea explained at once the remarkable non-nervous epithelial parts of 

 the tube, which become so conspicuous as we descend the vertebrate phylum, 

 and every part of this tube bears the same resemblance to various parts of 

 the C.lsr.S. as the dorsal stomach and intestine of an Arthropod. As a 

 crowning of his conception the pineal eyes fit into the right place of the 

 scheme ; and the resemblances become greater and more numerous on the one 

 hand in Ammocoetes, as was to be expected in the lowest available vertebrate, 

 and on the other in Limulus, the King crab. In short, there was now a 

 provisional working hypothesis, obtained by a direct logical process from the 

 consideration of the vertebrate nervous system. 



