262 TRANSITIONS AMONG THE CLASSES 



Now it would be impossible for me to, show all the details of 

 transition among these various groups, from one into another, 

 because, in the first place, time would not allow it, and secondly, 

 it could not be done except by the actual inspection of speci- 

 mens ; and even then the demonstration would have to be illus- 

 trated in a superficial manner. These transitions, in the majority 

 of cases, are such as are addressed to the eye and thought of 

 the well-studied observer of nature; and when they can be illus- 

 trated, they afford a view of only the more general relations. It 

 is on this account that, in a course of lectures like these, many 

 things are to be taken for granted upon the assertion of the 

 lecturer, whilst occasionally he is able to bring forward more or 

 less of his proofs in. detail. In the latter case, the presentation 

 of facts involves a form of special pleading which can hardly be 

 addressed to an audience which is not directly and practically 

 interested in the subject. The dryness of the details are beyond 

 the patience of those who, at most, look forward to the final 

 exhibition of the more general results. 



It would be extremely onerous, for instance, to attempt to 

 show in what points certain Fishes and certain Reptiles cannot 

 be distinguished from one another, and in what points they 

 essentially or manifestly differ. We can only say, for instance, 

 that certain kinds of Catfish or Horned-pouts have an organi- 

 zation so much like that of certain Salamanders and frog-like 

 Reptiles, that naturalists class the two together in one group ; 

 and that not only are they related in this most general way, but 

 there are among them certain forms which cannot be definitely 

 and unequivocally proved to be either Fish or Reptile ; and on 

 this account naturalists are all the more inclined to look upon two 

 groups which have no definite boundary line as merging into one 

 another. Here is an instance in illustration of this relation. 



This creature (fig. 169) has a fish-like form, but so have the 

 tadpoles of Salamanders, and certain adult Salamanders. Its 

 body is covered with scales, whose structure, however, is not like 

 that of the scales of ordinary fishes, but rather of the Lizards 

 (fig. 153). It has internal gills like those of a fish, but a part 



