MATERIALS FOR A MONOGRAPH OF THE ASCONS. 549 



triaxon spicules of the Hexactinellids ; and Dreyer is obliged 

 to leave them entirely out of consideration. 



In considering these "different views, certain objections to 

 each one of them readily suggest themselves. 



Haeckel's theory is entirely borne out as regards the mo- 

 lecular structure of the spicules by Ebner's investigations 

 (1887), and this author proposes to retain for them Haeckel's 

 term '* biocrystals." But crystallisation cannot be taken as an 

 adequate explanation of the external form of the spicules, at 

 least as they now occur. For in the case of the monaxons, 

 Haeckel's " absolutely regular spindle " is a form which never 

 occurs, and though it might possibly have been the form under 

 which the spicules first appeared at a very remote period of the 

 earth's history, yet Haeckel himself is of opinion that the 

 monaxon spicules as we know them are the results of natural 

 selection acting upon the primitive biocrystal ; in other words, 

 it is natural selection, and not crystallisation, which is the 

 factor which models the various forms assumed by the monaxon 

 spicules at the present day. A similar train of reasoning, 

 strengthened by additional arguments, applies to the case of 

 the triradiate spicules, and insuperable objections to Haeckel's 

 theory in this instance are furnished, it seems to me, by the 

 observations upon the origin of these bodies set forth above by 

 me. The case against Haeckel may be summed up as follows : 



1. Ebner has already pointed out (1887, p. 134) that the 

 morphological axis of a triradiate spicule cannot possibly be 

 compared to a crystalline axis. 



2. The occurrence in other sponges of forms of spicule even 

 more symmetrical and regular than those of Calcarea, com- 

 posed of non-crystalline materials, such as colloid silica or 

 spongin, has supplied such a weighty argument to all Haeckel's 

 critics, that to attack his theory on these grounds may seem to 

 many like flogging a dead horse. Since, however, it is clear 

 that in other cases the form of the spicule is independent of 

 the nature of the material, it is reasonable to suppose that the 

 same may be true of calcareous spicules. 



3. And, finally, ray own investigations prove, to my mind, 



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