SPONGIA. 165 



The recent observations of Mr Bowerbank have shown that 

 this description of the Spongise is erroneous. He has proved 

 that the " filament" is solid ; and he has also proved that it is 

 often abundantly furnished with siliceous spicula. * It may be 

 said that the latter species are properly members of the genus 

 Halichondria, but so similar are they in appearance, form, struc- 

 ture, elasticity and bibacity to the purely horny or keratose kinds, 

 and so imlike the tj'pical siliceous ones, that the separation 

 would be injurious to a natural arrangement, and would be hos- 

 tile to the maxim " that a genus should furnish a character, 

 not a character form a genus." f Indeed it is too evident that 

 the distinction between the keratose and the siliceous sponges 

 is one of degree only, not of essence : in the former the fibre is 

 either entirely horny or it secretes minute spicula which are 

 always imbedded in the centre of the tissue, — in the latter the 

 animal matter has a greater secerning power, and in general 

 the spicula lie exposed or predominate so far as to constitute 

 the principal ingredient of the sponge. Yet there are species 

 which commingle the characters of Halichondria and Spongia 

 so intimately in their structure that we can at best but puzzle 

 out their true genus. 



Mr Bowerbank has also made the very interesting discovery 



* " The small fibres usually Lave none and the largest abound in them. 

 The young sponges are also frequently without them, but the adult ones 

 never." Bowerbank in litt. 



j- Linnceus " laid it down as a maxim, that all genera are as much 

 founded in nature as the species which compose them ; and hence fol- 

 lows one of the most just and valuable of all his principles, that a genus 

 should furnish a character, not a character form a genus ; or, in other 

 words, that a certain coincidence of structure, habit, and perhaps quali- 

 ties, among a number of plants, should strike the judgment of a botanist, 

 before he fixes on one or more technical characters, by which to stamp 

 and define such plants as one natiu'al genus." Sir J. E. Smith's Introd. 

 to Botany, p. 182. edit. 1833. 



