692 ON CONCRETIONS, SPICULES, ETC. [ch. 



currents; though even here, by the way, if I understand Haeckel 

 aright, he was thinking not of a direct or immediate physical 

 causation, but rather of one manifesting itself through the agency 

 of natural selection*. SoUas laid stress upon the "path of least 

 resistance" as determining the direction of growth; while Dreyer 

 dealt in greater detail with the 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 

 constituent cells, or by the minute structure of the intracellular 

 protoplasm. But neither of these writers, so far as I can discover, 

 was inchned to doubt for a moment the received canon of biology, 

 which sees in such strucHires as these the characteristics of true 

 organic species, the indications of blood-relationship and family hke- 

 ness, and the evidence by which evolutionary descent throughout 

 geologic time may be deduced or deciphered. 



Minchin, in a well-known paperf, took sides with F. E. Schultze, 

 and gave his reasons for dissenting from such mechanical theories as 

 those of SoUas and of Dreyer. For example, after pointing out 

 that all protoplasm contains a number of "granules" or micro- 

 somes, contained in an alveolar framework and lodged at the nodes 

 of a 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 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?" The answer is not far to seek. If the force which the 

 "mechanical" hypothesis has in view were simply that of mechanical 

 pressure, as between solid bodies, then indeed we should expect 

 that any substances lying between the impinging spheres would 

 tend to assume the quadriradiate or "tetraxon" form; but this 

 conclusion does not follow at all, in so far as it is to surface-energy 

 that we ascribe the phenomenon. Here the specific nature of the 

 substance makes all the difference. We cannot argue from one 



♦ Op. cit. p. 483. "Die geordnete, oft so sehr regelmassige und zierliche Zusam- 

 mensetzung des Skeletsy stems ist zum grossten Theile unmittelbares Product 

 der Wasserstromung; die characteristische Lagerung der Spioula ist von der 

 constanten Richtung des Wasserstroms hervorgebracht; zum kleinsten Theile ist 

 Bie die Folge von Anpassungen an untergeordnete ^ussere Existenzbedingungen." 



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



