THE PRESIDENT'S ADDRESS. 67 



be able to convince you that in the evolution of living organisms 

 themselves by-products have played a part not unlike that which 

 they have played in the evolution of industries. 



You have probably already began to wonder why I should 

 have chosen such a subject as this for an address to a micro- 

 scopical club ; but the reason will now become apparent, for 

 I propose to endeavour to elaborate the ideas which I have 

 been suggesting to you by reference to organisms which have 

 long been favourite subjects with the microscopist, and to 

 characters which can only be investigated with the aid of the 

 microscope. 



We shall perhaps find nowhere in the animal kingdom a more 

 exact analogy to the utilisation of waste products in human 

 industries than in the curious rotifer Melicerta janus. As 

 you are all aware, this minute but highly complex organism 

 builds for itself a beautiful dwelling-place out of pellets of its 

 own dung. I do not, however, propose to dwell upon such cases 

 as this, and for our present purposes I must ask you to allow 

 me to interpret the term waste products, or if you prefer it, by- 

 products for it is obvious that the two cannot be sharply 

 distinguished from one another in a less literal manner. 



There is, in my opinion, no group of organisms better suited 

 for the illustration of the fundamental principles of organic 

 evolution than the Sponges. This arises from the fact that they 

 combine with an essential simplicity of structure an inexhaustible 

 variation in detail, and that this variation is to a very great extent 

 clearly and precisely expressed in the form of the microscopical 

 calcareous or siliceous spicules of which the skeleton is ordinarily 

 composed. Moreover, it appears that an unusual number of 

 connecting links have been preserved to the present day, so that 

 we are able to trace beautiful evolutionary series in the wonderful 

 spicule -forms of existing species. 



Take, for example, the siliceous spicules which are so character- 

 istic of the Tetraxonida. These are probably all to be derived 

 from a primitive ancestral form or archetype (fig. 1) consisting 

 of four rays diverging at equal angles from a common centre, 

 like the axes which connect the angles of a regular tetrahedron 

 with its central point. The assumption of this regular geometri- 

 cal form by a non-crystalline substance like the hydrated silica, 

 or opal, of which these spicules really consist, is a very remarkable 



