Animals Before Man 



group of animals may stand considerably above 

 tlie level of the lowest species in the next, and 

 on the whole more specialized group, while 

 this in turn has some species above the grade 

 of its immediate successor, so that there is a 

 continuous overlapping of species. We have a 

 good examj)le of this in the moUusks, where 

 the squids and cuttlefishes are far more special- 

 ized and higher in structure than such sim23le 

 vertebrates as hagfishes and lampreys, although 

 these claim a place in, or next, man's own class. 

 And just as animals overlap in classification, so 

 that they can not be arranged in a straight line 

 from lowest to highest, just so they overlap in 

 point of time ; and while certain species may 

 be, and often are, confined within very definite 

 limits of time, as indicated by the rocks in 

 which their remains occur, the group of which 

 they form a paii; is apt to overstep these narrow 

 limits. So, for the sake of smoothness of narra- 

 tive, we may well let some chapters overlap a 

 little in order to treat the history of various 

 groups of animals in a more connected manner. 

 So, too, we may pass over the record of the 



74 



