Adaptations of Fishes 223 



inhibitive in Chologaster agassizii, which hves in caves and 

 develops well-formed eyes, it is evident that the causes con- 

 trolling the development are hereditarily established in the egg 

 by an accumulation of such degenerative changes as are still 

 notable in the later history of the eye of the adult. 



"The foundations of the eye are normally laid, but the 

 superstructure, instead of continuing the plan with additional 

 material, completes it out of the material provided for the 

 foundations. The development of the foundation of the eye is 

 phylogenic; the stages beyond the foundations are direct." 



Conditions of Evolution among Fishes. — Dr. Bashford Dean 

 ("Fishes, Living and Fossil") has the following observations on 

 the processes of adaptation among fishes: 



"The evolution of groups of fishes must accordingly have 

 taken place during only the longest periods of time. Their 

 aquatic life has evidently been unfavorable to deep-seated 

 structural changes, or at least has not permitted these to be 

 perpetuated. Recent fishes have diverged in but minor regards 

 from their ancestors of the Coal Measures. Within the same 

 duration of time, on the other hand, terrestrial vertebrates have 

 not only arisen, but have been widely differentiated. Among 

 land-living forms the amphibians, reptiles, birds, and mammals 

 have been evolved, and have given rise to more than sixty 

 orders. 



"The evolution of fishes has been confined to a noteworthy 

 degree within rigid and unshifting bounds ; their living medium, 

 with its mechanical effects upon fish-like forms and structures, 

 has for ages been almost constant in its conditions; its changes 

 of temperature and density and currents have rarely been more 

 than of local importance, and have influenced but little the 

 survival of genera and species widely distributed; its changes, 

 moreover, in the normal supply of food organisms cannot be 

 looked upon as noteworthy. Aquatic life has built few of 

 the direct barriers to survival, within which the terrestrial 

 forms appear to have been evolved by the keenest compe- 

 tition. 



"It is not, accordingly, remarkable that in their descent 

 fishes are known to have retained their tribal features, and to 

 have varied from each other only in details of structure. Their 



