MONOGRAPH OF THE EXISTING CEINOIDS. 13 



others, the interrelationships being in general accordance with Jordan's law; but 

 one species, always the most variable and the one occupying the position nearest the 

 center or general mean of the extremes of all the variable specific characters repre- 

 sented in the genus, will be found whose range, both geographical and bathymetrical, 

 is equal to the sum of the ranges of all the other species in the genus. 



Again, highly specialized species commonly occupy a specialized and circum- 

 scribed habitat, while generalized species are found among very diverse conditions. 

 Among the several species in a genus the one occupying the limits of the distri- 

 bution of the genus as a whole is as a rule the most variable in its characters, and 

 similarly in individual species the coefficient of variation among the individuals 

 increases in proportion to the distance from the center of distribution, primarily as 

 a result of existence under progressively increasing unfavorable or semipathological 

 conditions. 



There is a more or less apparent curious and significant exception to this rule, 

 however, for the center of distribution of a large group and the truth of the obser- 

 vation is, as a rule, greatly increased in proportion to the size and importance of 

 the group is marked by a most remarkable diversity in the individual, specific, 

 and generic characters of the organisms inhabiting the locality. This is the result 

 of an increase in the number of variants under optimum conditions a land of 

 incipient species formation and has no relation to the more or less pathological 

 type <5f variation seen along the outer edge of the habitat of a species or of a genus. 

 Association of species of a single genus or of related genera in pairs, each occu- 

 pying nearly or quite the same geographical and bathymetrical ranges, has fre- 

 quently been reported, cases occurring in most of the animal groups, and instances 

 of it appear among the crinoids. Some of these cases arc at once explained by the 

 difference in the breeding seasons of the associated forms which effectually prevents 

 any hybridization; but others are not quite so simple, although they may be 

 accounted for in various other ways. 



Not only are the crinoids plant-like in appearance and in the manner of their 

 existence, but some of them have, along with this curious superficial similarity, 

 acquired a more or less close correspondence in the comparative interrelationships 

 of their various systematic characters, just as have many of the arborescent marine 

 organisms. 



The degree of stability of the generic and specific characters and of the corre- 

 lation of the characters presented by the several sets of structures and organs among 

 the comatulids is, broadly speaking, inversely proportionate to the fixity of habit 

 of the adults, and therefore in general to the number of arms possessed by the adults. 

 In such groups as the Antedoninse, where the animals are more or less active and are 

 capable of swimming about, the generic and specific characters and the character 

 correlations are, as a rule, strongly marked and readily defined. Such specific or 

 generic intergradation as occurs (and specific and generic intergnulati<>n is by no 

 means uncommon) takes the form of a gradual and uniform change in all the char- 

 acters whereby exactly the same balance of correlation is at all times maintained; 

 but in the highly multibrachiate groups in which the musculature in the proximal 

 portion of the arm is greatly reduced, especially in those groups which are highly 



