Chap. XIII. 



EEVERSION. 



53 



Many animals have the right and left sides of their body 

 unequally developed : this is well known to be the case with 

 flat-fish, in which the one side differs in thickness and colour, 

 and in the shape of the fins, from the other; and during the 

 growth of the young fish one eye actually travels, as shown by 

 Steenstrup, from the lower to the upper surface. 57 In most flat- 

 fishes the left is the blind side, but in some it is the right ; 

 though in both cases "wrong fishes," which are developed in 

 a reversed manner to what is usual, occasionally occur, and in 

 Platessa flesus the right or left side is indifferently developed, 

 the one as often as the other. With gasteropods or shell-fish, 

 the right and left sides are extremely unequal ; the far greater 

 number of species are dextral, with rare and occasional reversals 

 of development, and some few are normally sinistral ; but cer- 

 tain species of Bulimus, and many Achatinellas, 58 are as often 

 sinistral as dextral. I will give an analogous case in the great 

 Articulate kingdom : the two sides of Verruca 59 are so won- 

 derfully unlike, that without careful dissection it is extremely 

 difficult to recognise the corresponding parts on the opposite 

 sides of the body ; yet it is apparently a mere matter of chance 

 whether it be the right or the left side that undergoes so singu- 

 lar an amount of change. One plant is known to me 60 in which 

 the flower, according as it stands on the one or other side of the 

 spike, is unequally developed. In all the foregoing cases the 

 two sides of the animal are perfectly symmetrical at an early 

 period of growth. Now, whenever a species is as liable to be 

 unequally developed on the one as on the other side, we may 

 infer that the capacity for such development is present, though 

 latent, m the undeveloped side. And as a reversal of develop- 

 ment occasionally occurs in animals of many kinds, this latent 

 capacity is probably very common. 



The best yet simplest instances of characters lying dormant 

 are, perhaps, those previously given, in which chickens and 



5 7 Prof. Thomson on Steenstrup's 

 Views on the Obliquity of Flounders : 

 'Annals and Mag. of Nat. Hist ' Mav 

 1865, p. 361. '' J ' 



58 Dr. E. von Martens, in 'Annals 

 and Mag. of Nat. Hist.,' March, 1866 

 p. 209. 



59 Darwin, ' Balanidae,' Eay Soc, 1854, 

 p. 499 : see also the appended remarks 

 on the apparently capricious develop- 

 ment of the thoracic limbs on the right 

 and left sides in the higher crustaceans. 



60 Mormodes ignea: Darwin, 'Ferti- 

 lization of Orchids,' 1862, p. 251. 



