456 ON CONCRETIONS, SPICULES, ETC. [ch. 



as between solid bodies, then indeed we should expect that any 

 substances whatsoever, lying between the impinging spheres, 

 would tend (unless they were infinitely hard) 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 substances involved 

 makes all the difference. We cannot argue from one substance 

 to another ; adsorptive attraction shews its effect on one and not 

 on another; and we have not the least reason to be surprised if 

 we find that the little granules of protoplasmic material, which 

 as they lie bathed in the more fluid protoplasm have (presumably, 

 and as their shape indicates) a strong surface-tension of their 

 own, behave towards the adjacent vesicles in a very different 

 fashion to the incipient aggregations of calcareous or siliceous 

 matter in a colloid medium. "The ontogeny of the spicules," says 

 Professor Minchin, "points clearly to their regular form being a 

 phylogetiefic adaptation, ivhicJi has become fixed and handed on by 

 heredity, appearing in the ontogeny as a prophetic adaptation.''^ 

 And again, " The forms of the spicules are the result of adaptation 

 to the requirements of the sponge as a whole, produced by the 

 action of natural selection upon variation in every direction^ It 

 would scarcely be possible to illustrate more briefly and more 

 cogently than by these few w^ords (or the similar words of Haeckel 

 quoted on p. 454), the fundamental difference between the 

 Darwinian conception of the causation and determination of 

 Form, and that which is characteristic of the physical sciences. 



If I have dealt comparatively briefly with the inorganic 

 skeleton of sponges, in spite of the obvious importance of this 

 part of our subject from the physical or mechanical point of view, 

 it has been owing to several reasons. In the first place, though 

 the general trend of the phenomena is clear, it must be at once 

 admitted that many points are obscure, and could only be discussed 

 at the cost of a long argument. In the second place, the physical 

 theory is (as I have shewn) in manifest conflict with the accounts 

 given by various embryologists of the development of the spicules, 

 and of the current biological theories which their descriptions 

 embody; it is beyond our scope to deal with such descriptions 



