86 HEREDITARY CHARACTERS 



Remarkable adaptive changes in appearance may be 

 found among marine forms. The young stages of many 

 fishes and other animals which live near the surface are 

 frequently transparent, thus escaping the observation of 

 their enemies. The young of flat-fishes (soles, plaice, dabs, 

 flounders, &c.) are all hatched out at first with bodies shaped 

 like those of ordinary round fishes, and swim upright. 

 Gradually they become flattened from side to side, and the 

 eye on one side travels to the same side as the other, so that 

 both eyes are on that side of the fish destined to be the upper. 

 A study of the gradual changes which bring about this 

 adaptation in each individual fish, suggests most strongly 

 that the characters of the race have been acquired by the 

 selection of small variations, and not by large mutations. 

 We find no gap in any part of the process of change : all is 

 gradual and regular. 



Among the Holothurians (sea-cucumbers) only one is 

 known that lives in the open sea. 1 Its anterior extremity 

 is expanded like a parachute, and the whole organism is 

 semi-transparent. It lives at the surface of the sea and 

 has lost the characteristic spicules. All the other Holo- 

 thurians live at the bottom and are not transparent, many 

 of them being most brilliantly coloured and conspicuous. 

 Among them there is a regular gradation between allied forms 

 with a full complement of spicules and those having none. 

 Chirodota has two kinds of spicules, Sigmodota has but one 

 kind, while Rabdomolga has none. There are connecting 

 links, which in some cases make classification difficult. 2 



The adaptation in the pelagic form is obvious, the change 

 to transparency being the most striking in the direction of 

 concealment. 



When we turn to plants, we find adaptation just as 

 prominent and striking as among animals. 



1 Bendy and Hindle, Linn, Soc. Journ. (Zoology), vol. xxx , October 1907. 



2 Ludwig, H., "Holothurians of the Albatross Expedition," Bull. Mus. 

 Comp. Zoo., Harvard, vol. xxiv., 1893 ; Mem. Mus. Comp. Zoo., vol. xvii., No. 3, 

 1894, p. 183. 



