DEVELOPMENT OF THE ANIMAL KINGDOM. 71 



all the MoUusca and a very large proportion of 

 the Artieiilata. In the Vertebrata, the lowest 

 class also is wholly aquatic. The arrangement 

 appears to be this — ^the basis of each line is a 

 series of marine forms ; the remainder consists of 

 a series designed to breathe the atmosphere and 

 live upon laud, these being all of improved organiza- 

 tion. The classification which this system implies 

 may be said to be transverse to all ordinary classi- 

 fications. The invertebrate, ichth\-ic, reptilian, or- 

 nithic, and mammalian characters are horizontal 

 grades, through which the lines pass, and where 

 they send off branches ; not separate and inde- 

 pendent divisions. In any of these branches 

 where we have a clear knowledge of the various 

 forms, it is possible to trace the affinities, in con- 

 junction with an improved organization, through 

 genera which are adapted to a partially marine life, 

 to a residence in the mouths of rivers, or on shores 

 and muddy shallows, then through genera which 

 are, in succession, appropriate to marshes, jungles, 

 dry elevated plains, and mountains. And it is 

 this series of external conditions and adaptations 

 which has caused that system of analogies between 

 various families of animals which has of late at- 

 tracted attention. But the iumiediate cause of the 



