172 MKRISTIC VARIATION. [PARTI. 



('"in meat on tJie foregoing cases. 



In tin- cases preceding many will no doubt see manifest examples of 

 !: version. Tli'i'- U . in which this view must be true, for it can 



s.-.-uv.-ly be questioned tli;it if we had before us the phylogenetic series 

 through \\hi.-h ill.- Flat ti>li.-s, the Narwhal, ic. are descended, it would 

 be seen ili.it each did at some time have a bilaterally symmetrical 

 an. -.-tor. Hut. f, , r all that, in an unqualified description of the change 

 at i reversion tin- siirniticance of the facts is missed. By the state- 

 ment that a LMV.-II variation is a r>-\ crsion it is meant that in the vary- 

 ing individual a form, once the normal, n -appears. The statement more- 

 over is e.-p.-ciallv intended to imply that the definiteness and magnitude 

 of the .--t.-p t'min normal to variety is due to the circumstance that 

 thi- varietv was mice a normal. It is meant, in fact, that the great- 

 tie,, of the modern change can be explained away by the suggestion 

 that in the pa>t, the form now presented as a variation, was once built 

 up I'V a gradual evolution, and that though in its modern appearance 

 there is Discontinuity, yet it was once evolved gradually. 



Now the attempt to apply this reasoning, especially to the case of 

 the "double" Flat-fishe-, leads to difficulty. We may admit that in so 

 far as the varietio are bilaterally symmetrical they represent a normal. 

 Their bilateral symmetry, as a quality apart, may be an ancestral 

 character, if any one is pleased so to call it. But that in the contem- 

 porary resumption of a I >ilateral symmetry we have in any further sense 

 a reappearance of an ancestral form is very unlikely. 



I irst it might be fairly argued that it is improbable that there was 

 ever a typical Ilat-tish ha\in.L, r <>n finfh sides the peculiar pigmentation 

 of the PI-CM -nt upper sides of the Pleuronectida? of our day. Such a 

 creature would !> highly anomalous. But even if in strictness we 

 forego the assumption that since the evolution of Flat-fishes there has 

 never been an ancestor fully pigmented on both sides, there still 

 remains the dilliculty that each species may in the "double" state have 

 upon its lower side the >/.///.- colour proper to its own upper side. A 

 notable instance of this has been mentioned in the Plaice (p. 467); and 

 here not only was the pigmentation of the lower side, as far as it went, 

 like that of the upper, but the spots were even almost bilaterally 

 symmetrical. It is true that the lower side does not in every case copy 

 the upper in colour, but it HHII/ do so; and, in proportion as it does so 

 in different species, so far at least are the changes not simply revers- 

 ions ; fur the several patterns of Turbot, Plaice fcc. are mutually ex- 

 clusive and it can hardly be supposed that each species had separately 

 a "double"' ancestor having the present specific pattern on both sides. 



The outcome of this reasoning is to shew that the hypothesis of 

 Reversion in the strict sense is an insufficient account of the actual 

 variation in these Flat-fishes, and in the production of these varying 

 forms there is thus a Discontinuity over and above that which can be 

 ascribed to Reversion. The facts stated in connexion with the Plaice 

 (p. 4G7), especially the symmetry of the spots, probably indicate the real 

 nature of this Discontinuity, and raise a presumption that in the new 

 resemblance of the lower side to the upper we have a phenomenon of 

 Symmetry resembling that Homoeosis shewn to occur between parts in 



