CEPHALOCHORDA. 361 



II. NOTE ON MERISTIC VARIATION IN THE GROUP. 



(By R. C. Punnett.) 



The Cephalochorda are probably regarded by the majority of zoologists as a form of 

 life which has remained comparatively unchanged during lengthy epochs of time. And although 

 from the nature of the case direct palaeontological evidence is lacking, morphology would 

 seem to support the view that the group is one of great antiquity. Moreover from the paucity 

 of species in existence at the present day and from the similarity of their structure, the 

 zoologist is led to conclude that the group has probably lost the power of extensive further 

 differentiation and is, as it were, on the down grade. In other words it seems natural to 

 regard the existing Cephalochorda as the relics of a great group which flourished in remote 

 geological ages, and which, owing to competition with higher forms, or to environmental 

 changes, or to some other reason, is represented to-day by a few species only. The inter- 

 esting question then arises to what extent a group, such as the Cephalochorda, which has 

 persisted from remote antiquity, has acquired a fixity of type, and to what extent it may 

 be supposed to have attained to a position of organic stability in which it is incapable of 

 giving origin to new species under altered conditions. If we admit the existence of Natural 

 Selection as a factor in evolution the problem resolves itself into the question whether the 

 variations exhibited are of sufficient magnitude and frequency to allow of the conception of 

 this factor coming into operation. 



To consider only the feature of meristic variation, most of the Vertebrata, and especially 

 the lower forms, exhibit considerable variation in the number of segments of which the body 

 is composed. In certain other vertebrates again the number of segments is nearly constant. 

 Especially is this the case among forms which have reached a high pitch of specialization, 

 such as the mackereP among fishes, the Anurous Amphibia, and many birds and mammals. 

 Accompanying this high degree of specialization there would seem to be a loss of plasticity 

 in the meristic series of which the animal is composed. To what extent is this fixity of 

 form to be found among groups of great antiquity as well as among those of high 

 specialization ? 



Amphioxus would seem to afford an admirable object for consideration in this connection, 

 and with regard to the amount of variation it shews there is some difference of opinion. 

 In his recent work on Nautilus Willey- has referred to Amphioxus as " another example of 

 a relatively fixed type which presents little range of variation." On the other hand Kirkaldy' 

 has shown that the number of myotomes in all the species known is subject to considerable 

 variation. In this connection it must be mentioned, that although the range of variation 

 may be of not inconsiderable extent, the species may yet be a comparatively non-variable 

 one. The number of specimens which group themselves closely round the mean may be very 

 large, whilst those which lie on either side of the mean may be numerically very few. In 



1 Williamson, H. C, Eighteenth Report of Scottish - Willey, A., Zoological Eesxdts, Pt. vi. 1902 (p. 811). 



Fishery Board, 1899. ^ Kirkaldy, J. W., Quart. Journ. 3Iic. Sci., 1895. 



