ON THE YOUNG STAGES OF ECHINI. 747 



In the Spatangoids the apparently sudden transitions, resulting from the 

 more rapid growth while passing from a stage of comparative rest to a suc- 

 ceeding stage of slow growth, are best shown in the development of Echino- 

 lampas (PI. XVI.). The stages this genus passes through extend over a 

 very great range, and suggest affinities and identities previously unsuspected. 

 We can trace the mode of transition of the simple ambulacra of Galeritidae 

 and of Echinoneus to the petaloid ambulacra of the majority of Spatangoids. 

 the mode of formation of the bourrelets from an eminently Clypeastroid 

 actinostome, and the changes which transform the simple radiating pores of 

 the ambulacra! system round the actinostome to the more complicated 

 phyllodes of the Petalosticha ; showing that stages we have been accustomed 

 to consider as emphatically distinct, when compared as adults or as occurring 

 in distinct genera, may be brought about by very slight modifications of 

 apparently little consequence at the time of their origin in the growth 

 of an individual, but which, proceeding very rapidly, end in producing 

 marked changes before they reach the next period of rest. 



By making a similar comparison between the extremes as known from 

 recent and fossil species of Echini and intercalating intermediate recent and 

 fossil forms, the conclusion has been drawn that we must eventually find all 

 or most of the intermediate unknown forms between apparently disconnected 

 types, if the theory of evolution be correct, because so many of the missing 

 links have been found in our more recent explorations both on the land and 

 in the depths of the sea. Such a conclusion seems to me unwarranted. On 

 the contrary, judging from the analogy of the development of living species, 

 we can only say that the genesis of the species may have been homologous 

 to the development of the individuals as we know it from embryological 

 data, and that we shall always find gaps in the fossil series (even were they 

 all known) which cannot be filled on the supposition that there is a genetic 

 connection between them. 



