ORGANIC FORM AND ARCHITECTURE 68i 



special term for this, since they are all fishes. What is illustrated 

 is merely a similarity of adaptation among related forms. But it is 

 different when the whales are brought into the comparison. This is 

 homoplasty or convergence, for the similarity of adaptation occurs 

 in forms that are in no way related, whales being mammals and thus 

 very remote from fishes, as Aristotle clearly discerned over two 

 thousand years ago. . 



It is very instructive to place in front of a student or oneself: 

 (i) a burrowing Amphibian (say a Caecilian like Ichthyophis), (2) a 

 burrowing lizard (say an Amphisbaenid or a slow- worm), and (3) a 

 burrowing snake (say Typhlops), for the three specimens are super- 

 ficially much alike in their worm-like shape. Yet a slow- worm is 

 a lizard or Lacertilian, structurally very different from a snake or 

 Ophidian, while the Caecilian is far away from both of the others, 

 being an Amphibian with, for instance, gills in an early stage of its 

 development. 



Sometimes the convergence is very superficial, for swallows and 



Fig. 96. 

 Deep-sea Cuttlefish, with "telescope eyes." After Chun. 



swifts, which belong to very different orders of birds, are not very 

 like one another except to the careless eye, and no one could really 

 confuse a dolphin and a shark. Yet many serious errors in classi- 

 fication have been due to mistaking convergent resemblance for 

 genuine homology, and this mistake ceases to be surprising when 

 we examine convergences not between entire animals, but between 

 parts. Thus there is a very striking detailed resemblance between 

 the skull of an ordinary carnivore like a wolf and the skull of a 

 carnivorous marsupial such as the Tasmanian Devil. Yet the 

 resemblances concern adaptations (in teeth and crests and other 

 features) which must have been evolved quite independently m the 

 two cases. Similarly, the marsupial Wombat has its Rodent-like 

 features and the "marsupial mole" its mole-like features. 



But homoplastic resemblance is often much subtler than in the 

 examples we have given, and the "telescope eyes" of some Deep 

 Sea fishes and Cephalopods may be taken in illustration. Telescope 

 eyes" are elongated into cylinders, projecting like opera-glasses on 

 the top of the head, with the lens relatively large and at a greater 

 distance than usual from the surface of the retina on which the 

 image is formed. These peculiar eyes are adapted to make the most 



