212 ANIMAL PEDIGREES 



more regular ancestors. The young Clypeastroid 

 for example has an ovoid test, a small number of 

 coronal plates, few and large primary tubercles and 

 spines, simple straight ambulacral areas, and no 

 petaloid ambulacra ; in fact has none of the 

 characteristic features of the adult Clypeastroid, 

 while the characters it does possess are those of 

 geologically older and preceding forms. So again, 

 in the group of Echinidae, the members of the com- 

 paratively recent polyporous group, in which each 

 ambulacral plate bears more than three pairs of 

 ambulacral pores, commence their existence in the 

 older and more primitive oligoporous condition, and 

 become polyporous through fusion of originally dis- 

 tinct ambulacral plates. 



Agassiz gives many other examples, and from 

 a careful consideration of the entire group, arrives 

 at the conclusion that " comparing the embryonic 

 development with the palaeontological one, we find 

 a remarkable similarity ; " and again, " the com- 

 parison of the Echini which have appeared since 

 the Lias with the young stages of growth of the 

 principal families of recent Echini, shows a most 

 striking coincidence, amounting almost to identity, 

 between the successive fossil genera and the various 

 stages of growth." 



In this connection Agassiz makes a suggestion of 

 much interest. We are apt he says to assume, 

 and perhaps rightly, that enormous periods of time 

 have elapsed during the conversion of genus into 

 genus, but the fact that these very changes can be 

 repeated before our eyes in a few days' or even 



