STRUCTURAL MODIFICATIONS 129 



is also responsible for forming the ventral surface of the 

 bilateral Echinoids and the deep-sea Holothurians. 



Neither have the Vertebrates escaped attitudinal changes. 

 Amphioxits, the soles, turbots, dabs, and other flat-fish classed 

 as Pleuronectids, remain on their sides. They therefore become 

 d^s-symmetrical (much like the molluscs that live under the 

 same conditions) , and carry their two eyes on the same side of 

 their body. The Echeneididae attach themselves to sharks, press- 

 ing against the body of their hosts their dorsal surface, which 

 thus functions as the ventral surface of other animals with 

 respect to light and the soil. The dorsal surface becomes 

 discoloured and flattened, while the ventral side takes on the 

 characteristic of the ordinary dorsal surface. The influence of 

 external conditions on form and colour is thus clearly shown. 



This influence is seen to operate unceasingly directly we make 

 any attempt to correlate animal characteristics with the con- 

 ditions in which they live, rather than considering them apart 

 from all the causes which, with any degree of plausibility, can 

 be invoked to explain their existence — as if they were the result 

 of some miracle. Let me give some examples. The links 

 which unite the main divisions of the Fishes x can be summed up 

 in this one. proposition : The branchial region, situated 

 between the head, to which the water offers resistance when 

 the fish swims, and the body, which is pushed forward by the 

 sudden propulsions of the tail, is progressively shortened 

 till it finally becomes atrophied in the Batrachians, their 

 descendants. Among the terrestrial vertebrates a neck, 

 which may be enormously elongated, takes the place of the 

 branchial region that has disappeared ; but when these 

 vertebrates again become aquatic and swim after the manner 

 of fishes, their neck, placed in the same mechanical conditions, 

 undergoes the same reduction, whether we take Reptiles like 

 Ichthyosaurus of the Secondary Epoch ; the Herbivora that 

 have become aquatic, like the Sea-cows and Dugongs ; Seals 

 that have become divers like the Zeuglodonts, or Cetaceans 

 which are probably descended from another stock. This 

 repetition of like phenomena, under like conditions, among 

 different Vertebrates, which, moreover, have preserved the 

 characteristic organization of their group, illustrates well how 

 these phenomena have been due to external actions modifying 



1 XLIII, 2469. 



K 



