1895.] Dr Gaskell, The Origin of Vertebrates. 19 



(3) The Origin of Vertebrates. By W. H. Gaskell, M.D., 

 LL.D., F.R.S., Fellow of Trinity Hall. 



The problem of the ancestry of the Vertebrates can be attacked 

 from the side of the Invertebrates or from that of the Vertebrates. 

 To my mind the best chance of a successful solution is to be found 

 in the study of the Vertebrates themselves, and especially of the 

 variation which has occurred in each organ or group of organs 

 within the Vertebrate phylum. 



The principles upon which I have investigated the problem 

 are based upon the continuity of the evolutionary process and 

 may be stated as follows — 



1. Whenever it is possible to state that the variation of an 

 organ or set of organs within the vertebrate phylum has taken 

 place in a well-defined direction, then, by tracing that same curve 

 of variation a step further, we must arrive at the condition of the 

 organ or set of organs in the immediate ancestor of the Ver- 

 tebrate. 



2. The same principle holds good whether the curve of 

 variation is an ascending or descending one, i.e. whether the 

 organ in question is becoming more important and more con- 

 spicuous as we ascend the vertebrate phylum, e.g. the cerebellum, 

 or becoming less important and degenerate, e.g. the pineal eye 

 and ganglion habenvlce. In this preliminary paper I propose to 

 give a short sketch of the history of all the most important organs 

 and tissues which I have investigated up to the present time and 

 to shew that in the case of all of them the phylogenetic signpost 

 points directly to the same group of animals, viz. the Limulus and 

 its congeners, as the ancestor of the Vertebrates. When once the 

 clue has been found then embryology confirms it and illustrates it; 

 of itself it is a bad guide but affords an excellent illustration of 

 what phylogeny teaches. 



To illustrate my meaning I will rapidly sketch the downward 

 path of the central nervous system and point out the conclusions 

 to which such a study directly leads. 



The history of the Vertebrate central nervous system. 



The highest vertebrate nervous system possesses large cerebral 

 hemispheres, a large cerebellum, a small pineal gland, choroid 

 plexuses which are pushed into the ventricles and a small mem- 

 branous roof to the IVth ventricle. As we descend the dorsal 

 part of the brain becomes more and more membranous, the 

 cerebral hemispheres are more and more confined to the ventral 



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