530 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1915. 



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 O. N. S. appears embryonically as a 

 single unsegmented tube. Here then was the next question forced upon Gaskell's 

 attention. Can not the two opposing views be reconciled by the suggestion 

 that the vertebrate C. N. S. consists of two parts, closely entangled, viz, a seg- 

 mental 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 nonnervous 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. N. 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 Am- 

 mocoetes, as was to be expected in the lowest available vertebrate, and on the 

 ether in Limulus. the King crab. In short, there was now a provisional work- 

 ing hypothesis, obtained by a direct logical process from the consideration of 

 the vertebrate nervous system. 



After this working explanation of the tubular nature of the C. N. S. the next 

 step was the inquiry into the nature of the cranial nerves and therefore the 

 double segmentation of the vertebrate body in the head region. Now he was in 

 the midst of the most complex and abstruse problem of morphology, involving 

 every organic system. The resemblances between Arthi'opods and vertebrates — 

 with Limulus and Ammocoetes as the champions — are, indeed, numerous and in 

 many cases perplexingly close. Of course, the more Gaskell became absorbed by 

 his research, the more resemblances he saw, many of which are in all probability 

 mere coincidences, or even erroneous. With great intuition and ingenuity he 

 connected them, and in some of the most important cases his argumentation as 

 to their being homologous structures has remained intact. He knew that if but 

 a few are true homologies, his case would be proven, according to all the ac- 

 cepted canons of the theory of descent, and all the rest could be waved aside 

 as incidental convergences, due to correlations, the possible laws of which we 

 are now only just beginning to speculate about. Hence he felt it necessary to 

 defend, so to speak, his whole extended line ; not that the yielding of some point 

 would menu a disastrous breach but because of the lack of criterion to know 

 which of his many points might pro^-e one of his best assets, viz, an absolute 

 homologue. 



On the other hand he felt justified in assuming as most unlikely that repre- 

 sentatives of two fundamentally different phyla should have produced so very 

 many close resemblances, so close in function, structure, and relative position 

 as to make it impossible to show them up as heterogeneous. He was also fully 

 aware of it that our time-honored conception of homologies versus analogies 

 and their application to phylogeny are under reconsideration. It is a blow to 

 the comparative anatomist and to the constructor of pedigrees, but all the more 

 interesting since it shows that it is life, function, adaptation, and inheritance, 



