566 E. A. MINCHIN. 



The ontogeny of the spicules, therefore, points clearly to 

 their regular form being a phylogenetic adaptation, which has 

 become fixed and handed on by heredity, appearing in the 

 ontogeny as a prophetic adaptation. Nowhere is this fact 

 more evident than in the rim of a growing oscular tube of 

 an Ascon, since in this region we find triradiate systems 

 being laid down and even attaining to their full size in a 

 perfectly regular manner before any pores are formed. At a 

 later period, as the gastral layer grows up, pores are formed 

 amongst and between them, but the spicules appear first, long 

 before the wall has the structure to which we find that their 

 form is an adaptation. This is true equally of the triradiate 

 systems of Clathrina with straight, and those of Leucoso- 

 lenia with curved rays, showing that neither form can be 

 explained as produced by functional excitation during the 

 ontogeny. 



The facts already mentioned with regard to the development 



of the embryo show that the hereditary impulse or tendency 



does not come into full play until the sponge as a whole has the 



structure to which the triradiate system is an adaptation, and 



that the spicules first formed are very irregular in consequence. 



But in spite of this influence exerted by the structure of the 



whole organism, it is not possible to connect the regularity of 



the spicules with any functional excitation in the ontogeny, 



though this is a possible explanation of the phylogeny. But 



at this stage of the inquiry we find ourselves confronted by a 



dilemma. If functional excitation produced the adaptation in 



phylogeny, then it is a case of an acquired character being 



inherited. If, on the other hand, acquired characters be not 



inherited, then the adaptation must have arisen from innate or 



constitutional variations,which became gradually fixed, doubtless 



by selection, as a constant hereditary tendency of the organism. 



If, therefore, we wish to avoid the hypothesis, not as yet 



established by a single clear instance, of the transmission of 



acquired characters, we are brought back, it seems to me, to 



the old familiar theory of natural selection, in order to explain 



the constant recurrence and regular form of the triradiate 



