30 THE PRESIDENT S ADDRESS. 



in the evolution of such remarkable forms as the extinct moa, in 

 which the wings had completely disappeared, and the surviving 

 kiwi, in which they have become reduced to mere vestiges. This 

 case is greatly strengthened by the fact that precisely the same 

 result has been arrived at by many birds belonging to quite 

 different families and in many widely separated islands. In 

 all these instances the same causes have, as was to be expected, 

 produced the same effect, and there is not the slightest ground 

 for supposing that mutation has had anything to do with the 

 matter. At the same time no zoologist would dream of main- 

 taining that the flightless kakapo, for example, is not specifically 

 distinct from the flying parrot from which it must have originated. 

 We can hardly believe that such species have arisen otherwise 

 than by slow, successive variation. 



I propose to-night, however, to seek evidence from the syste- 

 matic investigation of a very different group of animals. 



There is perhaps no phylum in the animal kingdom that can be 

 compared with the Sponges as regards the remarkable evolu- 

 tionary series exhibited by still surviving forms. I have already 

 dealt with one aspect of this phenomenon in a previous presidential 

 address, in which I had something to say about the evolutionary 

 series met with in the siliceous spicules of the Tetraxonida. 

 You will remember that in this group all the manifold forms 

 of spicule that occur can be derived from one and the same 

 fundamental type — the primitive tetract, with its four rays 

 diverging at equal angles from a common centre. By enlarge- 

 ment, diminution, branching, fusion, bending and so forth, of 

 one or more of the original rays, and sometimes by complete 

 suppression and sometimes by multiplication of rays, an endless 

 diversity of form has been produced, and the difl'erent forms 

 for the most part can be arranged in beautifully graduated series. 

 The same is true, to a lesser degree, of the other group of 

 siliceous sponges, the Hexactinellida, and also of the Calcarea, 

 in which the spicules are composed of carbonate of lime. Again, 

 in the case of the true horny sponges, or Euceratosa, we find a 

 beautiful evolutionary series leading up from the simply branched, 

 tree-like skeleton of Aplysilla to the complex reticulate skeleton 

 of the bath sponge. 



