8G M ERISTIC VARIATION. [PART i. 



become differentiated to form members of such non-coincident or 

 iii-1. -p. -inli-iit Meristic series. 



S. ,m.- \.-irsago 1 , in the course of an argument that Balanoglossus 

 -In. ul. I b.-'eon-id.-red as representing some of tlie ancestral characters 

 I ' ..rd.ita, I had occasion to refer to some of these difficulties, and 



iallv ti, tin- dim-rent characters of the two kinds of segmentation : 

 tli.it i-f t'he Annelids, in which the repetitions of the organs belonging 

 t.. tli.- -.\eral systems are coincident, and, on the other hand, that 



the Chordata, for example, in whicli this coincidence may be 



irregular or partial. At tliat time I was of opinion that these t\s<. 



f segmentation may, in certain cases, have had a different 



plr. ic history, and have resulted from processes essentially 



'. It appeared to me that we should recognize that, in the 



Amu-lids on the one hand, segmentation of the various systems of 



organs had been coincident from the beginning, while in the Chordata 



tin- -egmentation had been progressive and had arisen by segmentation 



<>r repetition of the organs of the several systems independently. The 



for this view were derived chieHy from the fact that it is 



il.lt- to arrange the lower Chordata in order of progressive segmen- 

 tation ,,f the several systems. In particular such treatment was shewn 

 to }> applicable to the central nervous system, the vertebral column 

 and tin- iiii->ol)lastic somites, and in these cases it was maintained that 

 tin- e\ i.|. -nee of the lower forms of Chordata goes to shew that segmen- 

 tati.m had occurred in these systems one after another, and that their 



Mentation was not derived from a form having a complete repetition 



H-li part in each segment: that these forms, in fact, shewed us the 

 history . .f this progress from a less segmented form to one more fully 

 segmented. 



Tin- \i.-us tlien s.-t forth have met with little acceptance. Those 

 \\lni an- occupied with the search for the pedigree of Vertebrates still 

 direct their in<niines on the hypothesis, expressed or implied, that in 

 the am-e-tral fnnn there was a series of complete segments, each 

 containing a r.-pre-entat i\ .- of each system of those organs which in 

 tin- present descendants appear in series. It is thus supposed that each 



'"nt ..f tin- priiuiti\e form must have liecn a kind of least common 

 denominator of the segments of its posterity. The possibility that the 



seg nt.ition .,f V. -i-t. -lii-atc -s may have arisen progressively is, indeed, 



.-a i c.-ly '-on.xidered at all. 



Though in tin- light of the study of Variation, it now seems to 

 ""' ''"at the discussion of these questions must be indefinitely post- 

 poned, and that there are radical objections to any attempt to interpret 

 the ta.-ts of anatomy and development in our present ignorance of 

 Variation, I ha\.- seen no reason to depart from the view expressed 

 '" ''" paper rcferr.-d to: that interpreted by the current methods of 

 morphological critieism, the facts go to shew that the segmentation of 

 ''" Choi-data dill'ers essentially from that of the Annelids t vc., and 

 that it has arisen by progressive segmentation of the several systems of 

 :m originally unsegm.-nted form. To those who hold as Dohrn, Gaskell, 

 N1 irshall and oth.-rs ha\.- done, that the evolution of Vertebrates has 



1 IJuart. -Jour. Mi,-r. Sri., 188G. 



