EVOLUTION OF TELEOSTS 



167 



and their character is often squamous. Scales are widely 

 specialized, thin, horn-like, ornate, overlapping their outer 

 margins, their inner rims set deeply but loosely in dermal 

 pockets (Fig. 31). Fins are dermal structures, their ancient 

 basal supports hardly to be distinguished ; the primitive 

 tail structure is so masked by clustered and fused skeletal 

 elements that its heterocercy is scarcely apparent. In 

 short, the most widely modified conditions can be shown 

 to exist in Teleosts in almost every structural character, 

 as in gills, teeth, opercula, circulatory and urinogenital 

 organs, sensory structures, and nervous system. They 

 have evidently been competing keenly in the struggle for 

 survival, for in every detail of form or structure the most 

 varied conditions exist. In addition to these structural 

 adaptations of Teleosts, changes in coloration have been 

 rendered possible by the transparency of their scales ; and 

 in their different families these changes have taken place 

 often with striking results : adaptive coloration, brilliant, 

 dull, mottled, inconspicuous, occurs with a range of varia- 

 tion which is not surpassed even by the colours of birds. 



It is not remarkable, therefore, that members of the 

 different groups of Teleosts should often parallel each 

 other in structural likenesses, when placed under the same 

 environmental conditions. Each organ, in fact, may be- 

 come a centre of variation, and confuse the line of the 

 descent of the minor groups ; for the keenest judgment 

 cannot select of all these varying structures those which 

 can definitely be made the standards of general comparison. 

 Environment, like a mould, has impressed itself upon 

 forms genetically remote, and in the end has placed them 

 side by side, apparently closely akin, similar in form and 

 structure. 



A striking instance of changes due to environment is 



