Stromatoporo'uh and of Eozoon. 457 



like Eozoon should have a calcareous skeleton and why other 

 Khizopoda should have a siliceous one. 



I think we must go back to the cooling mineral magma, 

 which would vary greatly in character over different areas 

 and which would form shall nver or deeper troughs. Probably 

 at first, before diffusion proJuced more or less uniformity, 

 seas filling those troughs wuild vary in composition. Hocks 

 are classiiied as acid or basic according to the percenta:4'e of 

 silicic acid present in them, and similarly a classification 

 based on mineral constituents might have been extended to 

 primitive seas (just as doctors classify mineral watery). 

 Sarcode living in seas with a high percejitage of silica and a 

 sni dl one of calcium salts would become saturated with ths^ 

 former mineral. Apparently silica forms a more intimare 

 union with protoplasm than carbonate of lime does, an 1, 

 though isotropic when thus U'lited, seems to impress some of 

 its mineral characters on that ))rotopla3m, as witness the 

 beautiful symmetry of Ridiolaia and of Hexactinellid 

 spicules*. Acanthin of Ridiolaria seems to be halfway 

 between silica-saturated protoplasm and silica of the skeleton. 

 Organisms with skeletons of silica, probably on ac-ount of 

 their want of flexibility, iiave only travelled a short distance 

 along the path of evolution. Tlie calcareous animal organisms, 

 on the other hand, have deposited their skeletal material in 

 masses (acicula, dorsal pillar, limb-girdles, &c.) ivhich have 

 sn-ved SLS points d^appui for contractile protoplasm, and thus 

 thev have been enabled to come into relation with a varied 

 enxironment. Consequently the path of evolution from the 

 Dawn Animal to human civilization has been along the 

 calcareous way. 



* For some time past I have thought that possibly sponges are tri- 

 phyletic and that there were at least three ancestral olynthuses (and not 

 one olytithus), viz. a shallow-water calcareous, a shallow-water pre-Demo- 

 spuige, and a deeper water Hexactinellid olyuthus, all three originating 

 from colonial Voloox-Yika Flagellates or Choanoflageilates. In all three 

 a free-swimming phase would be followed by a fixed one brought about 

 b}' the increasing disproportion between weight and carrying capacity. 

 On coming to rest the little organism would form a disk with an inferior 

 layer of granular cells aud a superior layer of flagellate cells. The 

 increasing growth of the lower nutritive layer would soon cause it to 

 encompass the upper motile layer, which would sink into it and an 

 olynthus would be formed. 



The Choauotiagdllate pre- Hexactinellid was highly vacuolated and its 

 delicate reticulate strands would be best sustained hj a type of spicule 

 having three axes crossing a common centre at right angles, like building 

 scaffolding, and the loose-textured protoplasmic network would permit of 

 the formation of large flagellated chambers. (I would add that I hold 

 this homoiousiou heresy very lightly.) 



Ann. (& Mag. N. Hist. 8er. 8. Vol. x. 31 



