Marvels of Pond- Life. 141 



classification of the infusoria has yet been devised, and 

 the life history of a great many is still very imperfectly 

 known. On the whole, the tendency of research is to 

 place many of them higher than they used to stand 

 after Ehrenberg's supposition of their having a plurality 

 of distinct stomachs, &c, was given up. Balbiani and 

 others have shown numerous cases of their forming 

 their eggs by a process analogous to that of higher 

 animals. Some really are, and others closely resemble, 

 the larval conditions of creatures higher in the scale, and 

 the contracted vesicle with its channel bears resemblance 

 to what is called the " water vascular system ,J of 

 worms. 



Zoological classification depends very much on mor- 

 phology, that is, the tracing of particular structures, 

 or parts, through all their stages, from the lowest to the 

 highest forms in which they are exhibited. In this 

 way the swimming bladder of a fish is shown to be a 

 rudimentary lung, though it has no respiratory func- 

 tions, and Mr. Kitchen Parker has found in the imper- 

 fect skull of the tadpole a rudimentary appearance of 

 bones belonging to the human ear. The comparative 

 anatomist, after a wide survey of the objects before 

 him, arranges them into groups. He asks what are the 

 characteristic things to be affirmed concerning all the A's 

 that cannot be said of all the B's; or of all the C's that 

 marks their difference from the A's or the D's. Careful 

 investigation upon these methods shows affinities where 

 they were not previously expected — birds and reptiles 

 being close relations, for example, instead of distant 

 connections — and they lessen the value for purposes of 



