EVOLUTION OF TELEOSTS 167 
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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 
