454 ON CONCRETIONS, SPICULES, ETC. [ch. 



not at all easy to determine) in relation to the tissues and chambers 

 of the sponge, it is at least clear that, whether they be separate 

 or be fused together (as often happens) in a composite skeleton, 

 they effect a symmetrical partitioning of space according to the 

 cubical system, in contrast to that closer packing which is repre- 

 sented and effected by the tetrahedral system*. 



This question of the origin and causation of the forms of 

 sponge-spicules, with which we have noAv briefly dealt, is all the 

 more important and all the more interesting because it has been 

 discussed time and again, from points of view which are charac- 

 teristic 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-crystalhsation"' ; he considered that these 

 " biocrystals " represented "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-crystalline 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 differen- 

 tiated in a vast variety of ways in the struggle for existence f." 

 What Haeckel precisely signified by these words is not clear to me. 



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

 developed whether the material were crystalline or non-crystalline, 

 abandoned all theories based upon crystallisation ; 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 walls of the sponge, and found 

 clear evidence of "utiUty" in the specific structure of these 

 skeletal elements. 



* Cliall. Rej). Heractinellida, pis. xvi, liii, Ixxvi, Ixxxviii. 



f "Hierbei nahm der kolilensaure Kalk eine halb-krystallinische Beschaffen- 

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

 mit einer geringen Quantitat von organischer Substanz zu jenen individuellen, 

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

 beniitzt, und spaterhin durch die Wecliselwirkung von Anpassung und Vererbung 

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

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



