70 Mr. G. P. Bidder. 



The young spicule of L. Lieberkuhnii is not a skeleton, but a com- 

 plete isosceles triangle (fig. 6), and the same is recorded by Breitfuss 

 (22) for the adult spicules of his new genus Sphenophorina. 



That natural selection has played a most important part in the 

 arrangement of skeleton is shown clearly by the schemes in the 

 Heteroccela, analysed by Polejaeff (7) and Dendy (17). And if 

 crystallisation of calcite had not been advantageous to the sponge, it 

 would have assumed no more importance than crystallisation of 

 urates among higher animals. But the strangely beautiful and 

 elaborate patterns of calcareous spicules are truly crystalline, and 

 their curves and angles, as they vary from one individual to another,* 

 are not necessarily of appreciable advantage or disadvantage to the 

 species in which they are found. And, in spite of the colloid nature 

 of silica, Schulze's beautiful figures of Hexactinellid spicules prove 

 to me that this substance has some property akin to crystallisation 

 on the cubic system, to which the triaxon forms of siliceous spiculep, 

 and probably the tetraxon also, are to be ascribed. It remains to be 

 shown whether the complex detail of a peacock's tail may not also- 

 be referable to a mathematical equation, rather than to the aesthetic 

 nicety of ancestral hens. De minimis non curat lex vitce. 



[I find that in attempting condensation of statement I have not 

 succeeded in completely explaining the position above advanced. 



There is probably no group in which, more clearly than in the cal- 

 careous sponges, change phylogenetically progressive can be shown 

 to result in progressive advantage to the organism, whether we con- 

 sider the canal-system or the arrangement of the skeleton. It is not 

 under dispute that, where the form of the spicule can appreciably 

 affect the chances of survival of a sponge, there natural selection 

 dictates to the necessary degree the form of the spicule. This it may 

 do either directly, as appears probably the case in many acicular 

 spicules, by developing a stubborn morphographic sense in the 

 skeletogenous cells ; or indirectly, by varying the chemico-physical 

 conditions of the secreted fluid, so that the form of crystallisation 

 changes. Such indirect action of selection in A. cerebrum may possi- 

 bly explain the presence of thorns on the gastral ray, though irre- 

 sponsible for their arrangement. 



But, while granting fully that the most apparently unimportant 

 variations may prove vital to the organism, it appears logically 

 necessary that there must be variations which are unimportant, or 

 unimportant compared with solid advantages with which they are 

 necessarily concomitant. In the case of calcareous triradiates the 

 solid advantage is that a mechanically useful skeletal element is 



of A. cerebrum, its crystalline position tends to give it a laminate rather than an 

 acicular form. 



* In Sycon raphanus this is most markedly the case. 



