BIOLOGICAL PALAEONTOLOGY 95 



the madreporite perforating them all. This morpho- 

 genetic progress can be traced in the phyletic sequence 

 Holectypoida -* Clypeastroida ; the two types of evolu- 

 tion are manifestly coincident and contemporaneous, 

 the former supplying true evidence of the course of 

 the latter. But in the other morphogenetic lines such 

 correspondence cannot be claimed. Although Anor- 

 /^^wj-^Spatangidae may be a phyletic sequence, 

 Anorthopygus -*> Conulus is improbable; while apical 

 systems more elongate than that of Echinocorys existed 

 long before Conulus appeared. The sequence illustrated 

 may well be that by which elaboration was attained ; 

 the examples used need not have been in a direct genea- 

 logical line. 



(II) POST-LARVAL ONTOGENY AND 

 MORPHOGENY 



Embryology is one of the most fruitful and fascinating 

 branches of biological research. Not only does it reveal 

 the essential and fundamental unity of all living things 

 by demonstration of their common origin from fertilized 

 ova ; but the process of ontogeny, or building of the 

 individual, is shown to follow definite courses which, 

 with sundry imperfections and irregularities, seem to 

 correspond with the evolutional history of the groups 

 concerned. Recapitulation is by no means an infallible 

 guide to phylogeny ; the brevity of the period during 

 which developments reminiscent of long - continued 

 specialization are achieved demands omission of many 

 stages ; the compressed epitome sometimes departs from 

 its true historical sequence, while new structures may 

 appear in adaptation to larval environment. But 

 embryological study is practically denied to a palae- 

 ontologist. Most larval stages are devoid of hard 



