MOJSrOGKAPH OF THE EXISTING CKINOIDS. 13 



Others, the interrelationslups 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 generaf 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. 



Agam, higlily speciahzed species commonly occupy a specialized and circum- 

 scribed habitat, wlaile generahzed species are found among very diverse conditions. 



Among the several species in a genus the one occupying the hmits of the distn- 

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

 similarly in inchvidual species the coefficient of variation among the individuals 

 mcreases in proportion to the cUstance from the center of distribution, primarily as 

 a result of existence under progressively increasing unfavorable or semipathological 



conditions. . i,- i 



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 mcreased 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 inhabitmg the locality. This is the result 

 of an increase in the number of variants under opthnum conditions— a khid of 

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

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



Association of species of a smgle genus or of related genera in pans, each occu- 

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

 quently been reported, cases occurrmg in most of the animal groups, and mstances 

 of it appear among the crinoids. Some of these cases are at once explamcd by the 

 difference m the breedmg seasons of the associated forms which effectuahy prevents 

 any hybridization; but others are not quite so shnple, 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 m the comparative interrelationships 

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



organisms. .„ , i j- .1 



The decTee 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 speakhig, 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 Antedonma?, where the anhnals are more or less active and are 

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

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

 aeneric mtergradation as occurs (and specific and generic mtergradation is by no 

 means uncommon) takes the form of a gradual and uniform change m aU the char- 

 acters whereby exactly the same balance of correlation is at dl tunes mamtained; 

 but in the highly multibrachiate gi'oups m which the musculature in the proxnnal 

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



