244 THE president's address. 



that the development of a siliceous sponge-spicule is a far more 

 complex process than is usually supposed, and that the course 

 of events is determined by a combination of physical and bio- 

 logical factors. The protorhabd, the formative cells and the 

 accessory silicoblasts all play their parts in bringing about the 

 final result. That result ajjpears to be primarily of a non- 

 adaptive, one might almost say accidental, character, and in 

 many cases the form of the spicule seems to remain entirely 

 without significance from the utilitarian point of view. Should 

 it happen to be useful for some particular purpose, however, the 

 sponge is not slow to make use of it, and in some mysterious 

 way arranges the spicules in the most suitable manner. In some 

 cases a useful form is no doubt fostered and improved under 

 the influence of natural selection, and that may be true of the 

 very remarkable discorhabds which we have been considering. 

 The perfect adaptation of the manubrium as an organ for the 

 fixation of the spicule in the fibrous cortex of the sponge certainly 

 suggests the operation of this factor, but it seems very impro- 

 bable that natural selection can have had anything to do with 

 the remarkable specific differences between the apices of these 

 spicules in Latrunculia hocagei and L. apicalis. Still less can 

 we suppose that natural selection is responsible for the difference 

 in the position of the median whorl in the two species, which 

 we have seen to be attributable to purely mechanical causes. In 

 short, we may, I think, conclude that the facts which I have 

 submitted for your consideration this evening afford consider- 

 able evidence in favour of the view that the characters by 

 which one species is separated from another are usually of little 

 or no importance to their possessor in the struggle for existence, 

 while adaptations are usually shared by many different species. 



Description of Plates 15-17. 



Figs. 1-7. Latrunculia bocagci Ridley and Dendy. 



Fig. 1. Protorhabd, with whorl of formative cells (/.c.) near the 

 middle, x 1,900. 

 ,, 2. Young spicule with two rings of formative cells (f.c.) 

 and apical and median thickenings commencing. 

 X 1,900. 



