IX] OF THE SKELETON OF SPONGES 455 



Sollas 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-currents. Though even here, by the way, if I under- 

 stand Haeckel aright, he was thinking not merely of a direct or im- 

 mediate physical causation , but of one manifesting itself through 

 the agency of natural selection *. Sollas laid stress upon the " path 

 of least resistance"' as determining the direction of growth; 

 while Dreyer dealt in greater detail with the various tensions 

 and pressures to which the growing spicule was exposed, amid 

 the alveolar or vesicular structure which was represented alike 

 by the chambers of the sponge, by the reticulum of constituent 

 cells, or by the minute structure of the intracellular protoplasm. 

 But neither of these writers, so far as I can discover, was inclined 

 to doubt for a moment the received canon of biology, which sees 

 in such structures as these the characteristics of true organic 

 species, and the indications of an hereditary affinity by which 

 blood-relationship and the succession of evolutionary descent 

 throughout geologic time can be ultimately deduced. 



Lastly, Minchin, in a well-known paper f, took sides with 

 Schultze, and gave reasons for dissenting from such mechanical 

 theories as those of Sollas and of Dreyer. For example, after 

 pointing out that all protoplasm contains a number of "granules" 

 or microsomes, contained in the alveolar framework and lodged 

 at the nodes of the reticulum, he argued that these also ought to 

 acquire a form such as the spicules possess, if it were the case that 

 these latter owed their form to their very similar or identical 

 position. "If vesicular tension cannot in any other instance cause 

 the granules at the nodes to assume a tetraxon form, why should 

 it do so for the sclerites ? " In all probability the answer to this 

 question is not far to seek. If the force which the "mechanical" 

 hypothesis has in view were simply that of mechanical pressure, 



* Op. cif. p. 483. " Die geordnete, oft so sehr regelmassige imd zierliche Zusam- 

 mensetzung des Skeletsystems ist zum grossten Theile luimittelbares Product 

 der Wasserstromung ; die characteristische Lagerung der Spicula ist von der 

 constanten Richtung des Wasserstroms hervorgebracht ; zum kleinsten Theile ist 

 sie die Folge von Anpassungen an untergeordnete aussere Existenzbedingungen." 



t Materials for a Monograph of the Ascones, Q. J. M. S. XL, pp. 469-587, 1898. 



