316 EMBRYOLOGY AND CLASSIFICATION. 



the Fish can no longer be confounded with the 

 young Bird or Reptile or Mammal, and the blood- 

 vessels of the latter will soon enter into such con- 

 nection with the parent as to distinguish it also 

 from the young Bird or Reptile which forms no 

 such connection. I will not pursue this trans- 

 formation farther in all its details, which would 

 require numerous figures to be well understood, 

 but briefly allude to a few facts proving still more 

 clearly the unity of plan prevailing throughout 

 the whole Vertebrate type. 



The young Skate up to the period already de- 

 scribed does not differ from the young Shark; 

 but when the fins make their appearance, though 

 exactly the same at first in both these animals, 

 one pair in the Skate presently grows larger 

 than the others, expanding upon the sides of the 

 body and extending towards the tail and towards 

 the front of the head. Thus the young Skate, 

 as it advances in life, leaves behind the Shark 

 character, common to both in their younger 

 state, but permanent only in the Shark, in which 

 the fins undergo no such change. This shows 

 beyond a doubt that the family of Skates is su- 

 perior to that of Sharks, — an inference which 

 is confirmed by the order of their succession 

 upon earth, the Shark family having preceded 

 that of the Skates, in geological times. But it is 

 uot only among the lower groups that such cor- 



