CLARK : THE STARFISHES OF THE GENUS HELIASTER. 



75 



probable existence of connecting links between cumingii and polybrachius 

 at the Galapagos makes it very unlikely that it is merely the environ- 

 ment and isolation which are at work there. It seems clear that natural 

 selection has been an important agent in the case of canopus at any rate, 



ANCESTRAL FORM with rrw,r«ce rays 



Diagram 6. 



To show the phylogenetic and geographical relationships of the species of Heliaster. 



for while it can be claimed, if they please, by those, who are " done with 

 meekly accepting the dictum . . . that when we understand all the 

 conditions of the life of an organism, then and only then are we entitled 

 to say of this or that character that it is not of life or death value," 1 



i Kellogg, V. L., Science, Nov. 16, 1906, p. 627. 



