CEPHALOCHORDA. 365 



The figures given in this table have been calculated from data given in the following 

 papers : — 



Pleuronectes. H. M. Kyle, Eighteenth Annual Report of the Scottish Fishery Board, 1899. 



Clypea. F. Heincke, Naturgeschichte des Herings, 1898. 



Necturus. H. Bumpus, Joum. Morph. vol. xil. 1897. 



Spinax and Scyllium from unpublished data collected by the writer. 



Considering first the case of Asymmetron, it will be seen that the variability as measured 

 by the standard deviation {a) is greater than that found in the Teleosteans and in Spinax, 

 being only excelled by that found in Scyllium and in Necturus. This however is not an 

 altogether satisfactory criterion since it does not take into account the fact that we are dealing 

 with series of segments in which the total number differs greatly in the different groups. 

 The mean number of segments is 66'7 in Asymmetron, whilst in Spinax^ it is about 65 and 

 in Necturus only 47. This difficulty can to some extent be eliminated by considering the 

 coefficient of variation (C.V.) instead of the standard deviation. When this is done it is .found 

 that the value for Asymmetron comes out higher than that for all the Elasmobranchs, being 

 only exceeded by the value for Necturus and for a local race of Baltic plaice. On the other 

 hand the value for Heteropleuron comes out somewhat lower than that for the Elasmobranchs 

 and for most of the plaice and herrings, and very much lower than the value for Necturus. 

 When however it is borne in mind that the Vertebrates here dealt with are probably among 

 the most variable species of the group it seems only fair to conclude that the Branchio- 

 stomidae form a highly variable group in respect of their meristic segmentation, and that 

 we must give up the idea of their being characterized by fixity of form, at any rate with 

 regard to this feature. 



Why then, it may be asked, are the species of Cephalochorda so few in number ? The 

 great number of off'spring would lead us to suppose that the Struggle for Existence must be 

 severe ; the meristic variability of the species hitherto investigated is considerable ; and lastly 

 we have no reason for doubting that the Principle of Heredity also holds good in this con- 

 nection. Moreover many of the species occur in great abundance. We cannot but ask ourselves 

 how it is that, under what seem at first sight such favourable circumstances for its opera- 

 tion, the factor of Natural Selection has not come into play and produced in the group a 

 far greater diversity of form than we meet with. Is it possible that Natural Selection now 

 plays a comparatively unimportant role in the history of the group as regards the character 

 of meristic segmentation ? 



In this connection it must not be forgotten that the life of the Branchiostomidae is 

 completed under two entirely different environments. We find at first a pelagic larva pro- 

 duced in enormous numbers, but in which the total number of myotomes is not reached 

 until towards the conclusion of this stage of its existence. It is therefore not easy to see 

 how the high death-rate at this period can affect the ultimate number of segments, i.e. 

 whether in this respect it is truly selective. Later we have to deal with an adult so 

 adapted through long ages to living in sand that the constancy and uniformity of its sur- 

 roundings would seem to reduce the struggle with the environment to a minimum. Again, 

 the greatest variability would hardly be looked for in a species which had been exposed for 



• In reckoning the number of segments in Sjiinux and in the Elasmobranoh tail region have been counted as a single 

 Scylluim each pair of "half vertebrae" which characterize segment. 



47—2 



