106 



carried on among the* cephalopods by Hyatt and others in 

 this country, and by Wurteu! termor, Bianco, Karpinsky and 

 others in Europe. Jackson has studied the pelecypods and 

 the Palaeozoic Echini, and Beecher the brachiopode and 

 trilobites. The principle on which this work is based is 

 embodied in the law first clearly enunciated by Eouis 

 Agassiz and Carl Yogt: that the stages in the cycle of indi- 

 vidual life can be correlated with the characteristics of the 

 adult in allied types which appear earlier in the geologic 

 record. According to the modern interpretation of such 

 phenomena, this correspondence indicates a genetic relation, 

 the later forms having descended from the earlier, and each 

 recapitulating in its own life-history, more or less perfectly, 

 the life-history of the group to which it belongs. 



Investigations in this direction require the study of young 

 and intermediate as well as adult individuals. Immature 

 forms may frequently be found associated with the mature 

 individuals in the strata, and thev can be obtained by 

 processes described in the next chapter. For the details 

 of the methods of investigation in Biologic Palaeontology 

 the student is referred to the works of Hyatt, Jackson, 

 Beecher, Clarke, Schuchert, J. P. Smith, and others. 



