10 FACTORS IN VARIATION. 



of palaeontology we have oixr Lingulas, the Nautilus and 

 other forms remaining ])ractitally unchanged, so in the 

 world of life to-day many organisms, largely by reason of 

 their complete adaptation to a certain type of environment, 

 remain invariable, the hereditary forces being more potent 

 than the factors which make for change ; , others, in 

 contrast, are protean in their plasticity, and still in process 

 of rapid evolution. 



In the '■ Origin of Species," Darwin definitely expressed 

 his belief in the inheritance of modifications caused in 

 domesticated animals through the use or disuse of certain 

 ])arts (Chapter V.) He also quotes an instance in free 

 nature which affords valuable evidence on similar lines, 

 natural selection not being lost sight of in the citation. 

 The instance is that of the development of the e3'es of the 

 Pleuronectidae or flat-fishes, A\hich in very early life are 

 situated opposite from each other, the Ijody being 

 symmetrical, but which in the adult flat-fishes- are found 

 close together on the upjier side of the head. Quoting 

 Malms observations, Darwin* states : " The Pleuronectidae 

 whilst very young and still symmetrical, with their eyes 

 standing on opposite sides of the head, cannot long retain 

 a vertical position, owing to the excessive depth of their 

 bodies, the small size of their lateral fins, and to their being 

 destitute of a swim-bladder. Hence soon growing tired, 

 the}" fall to the bottom on one side. Whilst thus at rest 

 they often twist, as Malm observed, the lower eye upwards, 

 to see above them : and they do this so vigorously that 

 the eye is pressed hard against the upper part of the orbit. 

 The forehead between the ej-es consequently becomes, 

 as could be plainly seen, temporarily contracted in l)readth. 

 On one occasion Malm saw a young fiah raise and depress 

 the lower eye through an angular distance of about seventy 

 degrees. We should remonber that the skull at this early 

 stage is cartilaginous and flexible,' so that it readily yields 

 to muscular action. . . . Judging from analogy, the 

 tendency to distortion would no doubt be increased through 

 the principle of inheritance. . . We thus see that 



the first stages of the transit of the eye from one side of 



* Darwin : Tlif Origin of Sptcics, titli cd., 188G, i^p. 186-188. 



