IX] OF HEXACTINELLID SPICULES 691 



vibrations in quartz rods, more rapid even than Dendy's hypothesis 

 would seem to require. But on the other hand, it is only to few 

 and even exceptional spicules that the theory would seem to apply. 



This question of the origin and causation of the forms of sponge- 

 spicules, with which ^we have now sought to deal, is all the more 

 important and all the more interesting because it has been discussed 

 time and again, from points of view which are characteristic of 

 very different schools of thought in biology. Haeckel found in the 

 form of the sponge-spicule a typical illustration of his theory of 

 "bio-crystallisation"; he considered that these " biocrystals " re- 

 presented something midway — ein Mittelding — between an inorganic 

 crystal and an organic secretion; that there was a ''compromise 

 between the crystallising efforts of the calcium carbonate and the 

 formative activity of the fused cells of the syncytium"; and that 

 the semi-crystalhne secretions of calcium carbonate "were utihsed 

 by natural selection as 'spicules' for building up a skeleton, and 

 afterwards, by the interaction of adaptation and heredity, became 

 modified in form, and differentiated in a vast variety of ways, in 

 the struggle for existence*." What Haeckel precisely meant by 

 these words is not clear to me. 



F. E. Schultze, perceiving that identical forms of spicule were 

 developed whether the material were crystaUine or non-crystaUine, 

 abandoned all theories based upon crystaUisation ; he simply saw 

 in the form and arrangement of the spicules something which was 

 "best fitted" for its purpose, that is to say for the support and 

 strengthening of the porous w^alls of the sponge, and finding clear 

 evidence of "utility" in the specific characters of these skeletal 

 elements, had no difficulty in ascribing them to natural selection. 



SoUas and Dreyer, as we have seen, introduced in various ways 

 the conception of physical causation — as indeed Haeckel himself 

 had done in regard to one particular, when he supposed the position 

 of the spicules to be due to the constant passage of the water- 



* "Hierbei nahm der kohlensaure Kalk eine halb-krystallinische Beschaffen- 

 heit an, und gestaltete sich unter Aufnahme von Krystallwasser und in Verbindung 

 mit einer geringen Quantitat von organischer Substanz zu jenen individuellen, 

 festen Korpern, welche durch die naturliche Ziichtung als Spicula zur Skeletbildung 

 benutzt, und spaterhin durch die Wechselwirkung von Anpassung und Vererbung 

 ira Kampfe urns Dasein auf das Vielfaltigste umgebildet und differenziert wurden." 

 Die Kalkschwdmme, i, p. 377, 1872; cf. also pp. 482, 483. 



