FROM THE EDDYSTONE GROUNDS TO START POINT. 473 



Bottom-deposit. Mobius and Biitsclili (No. 88) record E. acutus on stones and mud (0-50 

 fatlioms) and on sand and shell (48-50 I'atlioms), and Agassiz {Challenger) on coral bottom 

 (425 fathoms), on rock (630 fathoms), and on mud (1350 fathoms). Bell (No. 8) records 

 the species from 55 fathoms, 110 fathoms, and 500 fathoms from Green's Trawling Cruise. 

 500 fathoms is apparently Green's Station VII. (No. 33), where the trawl brought up "one 

 large boulder weighing about 100 lbs., and several smaller ones, subangular and resembling 

 those of the Boulder Clay." Koehler gives a number of records from both Monaco (No. 60) 

 and Caudan (No. 61) dredgings in the Bay of Biscay, the sjiecies being taken on rock 

 and gravel, on coral, on sand and muddy sand, and several times on mud (80-400 fathoms). 



A question of some interest is raised by the distribution of the 

 two species Echinus esculeniiis and Echinus acutus in the Plymouth 

 district, which forms a good instance of a phenomenon very frequently 

 met with. We here find two species of very similar structure, yet 

 at the same time showing perfectly definite and constant difference 

 in structure, living side by side upon the same grounds under what seem 

 to be the same conditions, and having apparently exactly similar 

 habits. The one species (in this case Echinus esculcnius) is very 

 numerous, whilst the other is much less so. The questions are often 

 asked, how have these differences of structure been brouoht about and 

 what purpose do they serve ? Does the slight difference of structure in 

 the less numerous species correspond to some slight difference of habit ? 

 or if this is not the case, how does it come to pass that the one species, 

 being apparently less successfully adapted to the prevailing conditions 

 than its more numerous competitor, survives at all ? 



In the present case it is, T think, possible to give a satisfactory answer 

 to these questions, based on a consideration of the distribution of the 

 two species. 



From the records given above it is clear that Echinus esculeniiis is 

 essentially a shallow-water species, which may be found on the shore or 

 down to depths of 100 fathoms. It is most numerous in the laminariau 

 and coralline zones. Echinus acutus, on the other hand, is essentially a 

 deep-water species extending to a depth of over 1000 fathoms, and 

 apparently most numerous from 50 to 500 fathoms. If we compare 

 the essential differences of structure we see that each species is specially 

 adapted to the conditions prevailing where it is most numerous, or as we 

 may express it, the conditions prevailing at its centre of distribution. 

 These differences of structure depend chiefly on the fact that whilst 

 Echinus acutus is protected by a comparatively small number of very 

 long spines, E. esculentus carries a large number of short and stout ones, 

 the latter condition being obviously more advantageous in shallow water, 

 where wave action is often violent, whilst it is not difticult to imagine 

 that the former, viz., a few very long spines, is of greater value in deep 

 water, where there is never any disturbance. Each species must be 

 regarded, therefore, as being specially adapted to the conditions prevail- 

 ing at its own centre of distribution, and it is at that centre ("centre of 



