76 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



that the number of rays, the amount of their fusion, and the size and 

 arrangement of the abactinal spines are characters of no value in the 

 struggle for existence, there can hardly be any question that the ability 

 to reproduce vigorous young, at an early period of life, would be a factor 

 of impoi-tance in the establishment of Heliaster on an isolated island. 

 As diminutive size, a small number of rays and their comparative free- 

 dom, and slender abactinal spines are youthful characters in Heliaster, 

 it is significant that we find them correlated in canopus with sexual 

 maturity. It can hardly be doubted that natural selection, aided by 

 isolation and the correlation of characters, has, by working on an in- 

 herently variable and plastic organism, been the cause of the evolution 

 of canopus, and I see no reason to question the probability that a similar 

 process is going on in the formation of two new species of Heliaster at 

 the Galapagos. 



