CLARK : THE STARFISHES OF THE GENUS HELIASTER. 73 



The Interrelationships of the Species, and the Factors which 

 have aided their development. 



There are few starfishes whose habitat is so exclusively littoral as that 

 of Heliaster, and there are not many genera, containing several species, 

 whose area of distribution is so circumscribed. For these reasons the 

 genus offers an unusual opportunity for the study of the influence of 

 environment and the effect of isolation. Although this study could 

 only be properly carried on in the regions where the Heliasters live, 

 nevertheless the examination of a large number of specimens suggests 

 certain conclusions which are worth noting. In the first place we see 

 there are four areas, which so far as our present knowledge goes, are dis- 

 tinctly separated from each other, where Heliaster occurs, namely: — West 

 Coast of Mexico and Central America ; West Coast of South America 

 from Ecuador to Chili, inclusive ; Galapagos Islands ; Juan Fernandez. 

 In each of the first three regions two species of Heliaster occur, and in 

 the fourth, one, but there is no species common to any two of the dis- 

 tricts. We have no means of knowing which species is nearest the an- 

 cestral form, but it seems almost certain that the species with the fewest 

 and least united rays are the most primitive. We are equally ignorant 

 as to the place of origin of Heliaster, but there can hardly be any ques- 

 tion that it was somewhere along the mainland coast. If these two 

 points are assumed, kubiniji must be the nearest to the original Heliaster. 

 We can see that as there are no nearly allied species on the western trop- 

 ical coasts of America to compete with it, this form might gradually spread 

 southward, while it would not be likely to extend north of Lower Cali- 

 fornia, as it would then come into competition with numerous other 

 Asteriidse. Whether Heliasters still occur on the coast of Colombia we 

 do not know, but whether they do or not is of no special importance in 

 this connection, for kubiniji does not range very far south of Mexico 

 and is therefore entirely isolated at present from its South American rela- 

 tives. These latter under the different environmental conditions south 

 of the equator seem to have developed a larger number of rays and to 

 have them more fully united, as we find in helianthus. By a continued 

 (though slight) increase in the number of rays, and a marked increase in 

 their coalescence, accompanied by the development of stouter, capi- 

 tate, abactinal spines, jjolybrachius has arisen. The origin of microbra- 

 chius is less clear, but its affinities with polybrachius are so much more 

 apparent than any with kubiniji, we are almost forced to believe that it 

 represents a return northward of short-rayed Heliasters, which owing to 



