NON-ADAPTIVE CHARACTERS 



421 



825 fathoms in the Malay Archipelago. In some respects this is 

 the most remarkable sponge that has ever been discovered. Its 

 form (Fig. 188) is absolutely unique and resembles rather that of 

 some graceful plant than those of other sponges. It belongs to a 

 group of sponges whose members occur 

 mostly in much shallower water and are by 

 no means distinguished by beauty or sym- 

 metry of shape. In spiculation, moreover, 

 and other minute anatomical features, it 

 exhibits no striking peculiarities ; indeed, so 

 closely does it agree with more ordinary 

 species of Esperiopsis that it has not as 

 yet been considered necessary to separate it 

 generically. How then can we account for 

 this wonderful form ? Can we say that it 

 is an adaptation to any special conditions of 

 the environment ? It hardly seems likely 

 that this is the case, for we know that the 

 relatives of this sponge get on well enough 

 with all sorts of other forms, for the most 

 part more or less irregular and ill-defined, 

 and we know of nothing in the conditions 

 under which it lives to make such a unique and 

 beautiful form especially advantageous. The 

 " Challenger" obtained no less than thirteen 

 specimens of this sponge at the same place, 

 and the form appears to be quite constant. 

 No doubt the undisturbed condition of the 

 at great depths favours symmetry of 



FIG. 188. Esperiofisia 

 challenyeri, X . 

 (After Ridley and 

 Dendy, in " Chal- 

 lenger'' Eeport.) 



sea 



growth in sessile organisms like sponges, 

 but why this particular and absolutely unique 

 shape, so different from anything else that 

 has ever been met with ? 



These are questions which we cannot 

 answer, and it must suffice to point out that such cases can hardly 

 be explained by the theory of natural selection. Many factors must 

 combine in determining the course of evolution of any particular 

 organism, and some of the characters which result from their 

 interaction may perhaps have no more direct relation to the 

 necessities of the organism in the struggle for existence than has 

 the colour of a pebble to its continued existence on the sea-shore. 



