460 ECHINODERMATA ASTEROIDEA chap. 



relationship, to be found in the class Asteroidea, there has been 

 great uncertainty both as to the number and limits of the orders 

 into which the class should be divided, and also as to the limits 

 of tlie various species. The difficulty about the species is by no 

 means confined to the group Echinodermata ; in all cases where 

 the attempt is made to determine species by an examination of a 

 few specimens of unknown age there is bound to be uncertainty ; 

 the more so, as it becomes increasingly evident that there is no 

 sharp line to be drawn between local varieties and species. In 

 Echinodermata, however, there is the additional difficulty that 

 the acquisition of ripe genital cells does not necessarily mark the 

 termination of growth ; the animals can continue to grow and at 

 the same time slightly alter their characters. For this reason 

 many of the species described may be merely immature forms. 

 In proportion, however, as the collections from various localities 

 increase in number and size, difficulties connected with species 

 will tend to disappear. 



The disputes, however, as to the number of orders included in 

 the Asteroidea proceed from a different cause. The attempt to 

 construct detailed phylogenies involves the assumption that one 

 set of structures, which we take as the mark of the class, has 

 remained constant, whilst others which are regarded as adaptive, 

 may have been developed twice or thrice. As the two sets of 

 structures are often of about equal importance it will be seen 

 to what an enormous extent the personal equation enters in the 

 determination of these questions. 



Where, as in Asteroidea, the internal organisation is very 

 imiform, the best method of classification is to take as our basis 

 the different methods in which the demands of the environment 

 have been met. It is in this way, we hold, that divergence of 

 character has been produced, for whilst species may differ in 

 trifling details, families and orders differ in points of functional 

 importance. The fact that one of the orders may have sprung 

 from several allied species instead of one may be admitted, and 

 at the same time the hopelessness of trying to push phylogenetic 

 inference into details asserted. 



Sladen, in his Monograph of the Asteroidea collected by the 

 " Challenger " expedition, took for the basis of his system the 

 presence or absence of distinct pavement-like marginal plates 

 along the edges of the arms and the restriction of the papulae to 



