BIOLOGICAL PALAEONTOLOGY 101 



out the Mesozoic and Cainozoic eras, steady improve- 

 ments, along various lines, were introduced, until at the 

 present day such forms as the Echinometridae may be 

 considered thoroughly efficient for rock-dwelling habits. 

 But soon after this course of evolution had been entered 

 upon, some groups of Echinoids reverted to the sand- 

 habitat of their ancestors. The specializations for reef 

 life initiated and elaborated by the one series would have 

 been useless, and even dangerous, for the other, so that 

 a gradual return to qualities like those of Palaeozoic 

 types was begun. If racial progress is to be measured 

 by the degree of success attained in adaptation of an 

 organism to its environment, Colobocentrotus of the 

 southern coral-reefs, and Echinocardium of the British 

 sand-flats, are equally progressive. But in structure the 

 Echinometridae are far more elaborate than Palaeozoic 

 Echinoids, while the Spatangidae, with all their com- 

 plexity, show many "Perischoechinoid" features. Morpho- 

 genetically the former family is progressive (anagenetic), 

 the latter regressive (catagenetic) ; in efficiency, both are 

 equal. The solution of such an anomaly is probably to 

 be found in recognition that choice of environment is a 

 character indicative of evolutional activity, no less than 

 structural specialization. The preference shown by 

 Liassic Irregular Echinoids for muddy or sandy 

 surroundings was an expression of their catagenetic 

 tendency ; morphogenetic regression followed inevitably 

 after that backward step. 



In the distribution of living organisms there is a 

 pronounced tendency for moribund or antiquated types 

 to avoid positions involving exposure to danger or com- 

 petition. Thus the stalked Crinoids, many Brachiopods, 

 and Pleurotomaria^ now exist in the sheltered seclusion 

 of deep-water, although their more vigorous ancestors 

 joined successfully in the struggle for life in littoral 



