IX] OF THE SKELETON OF SPONGES 693 



substance to another; adsorptive attraction shews its effect on one 

 and not on another; and we have no reason to be surprised if we 

 find that the Httle 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 phylogenetic adaptation, 

 which has becmne fixed and handed on by heredity, appearing in the 

 ontogeny ai 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 words 

 (or the similar words of Haeckel quoted on p. 691), the fundamental 

 difference between the Darwinian conception of the causation and 

 determination of Form, and that which is based on, and characteristic 

 of, the physical sciences. 



Last of all, Dendy took a middle course. While admitting that 

 the majority of sponge-spicules are "the outcome of conditions 

 which are in large part purely physical," he still saw in them "a very 

 high taxonomic value," as "indications of phylogenetic history" 

 all on the ground that "it seei^is impossible to account in any other 

 way for the"^ fact that we can actually arrange the different forms 

 in such well-graduated series." At the same time he believed that 

 "the vast majority of spicule-characters appear to be non-adaptive," 

 "that no one form of spicule has, as a rule, any greater survival- 

 value than another," and that "the natural selection of favourable 

 varieties can have had very Uttle to do with the matter*." 



The quest after lines and evidences of descent dominated morpho- 

 logy for many years, and preoccupied the minds of two or three 

 generations of naturahsts. We find it easier to see than they did 

 that a graduated or consecutive series of forms may be based on 

 physical causes, that forms mathematically akin may belong to 



* Cf. A. Dendy, The Tetraxonid sponge-spicule : a study in evolution, Acta 

 Zoologica, 1921, pp. 136, 146, etc. Cf. also Bye-products of organic evolution, 

 jQurn. Quekett Microscop. Club, xii, pp. 65-82, 1913. 



