44 THE president's address. 



tunity for tlie occurrence of another, and there is no reason to 

 suppose that mutation has had anything to do with it beyond 

 occasionally and accidentally providing the organism with 

 some new character capable of being developed- into something 

 of value. 



In my opinion the slow, successive variations of the Dar- 

 winian theory have had far more to do with the evolution of 

 sponges than the process of mutation, and are mainly respon- 

 sible, under the guidance of natural selection, for adaptive modifi- 

 cations. 



This is perhaps nowhere better seen than in the root-tufts of 

 anchoring spicules so characteristic of those species of Tetillidae 

 that live on muddy sea-bottoms. The starting-point for the 

 development of such a root-tuft is, of course, provided by the 

 anatriaenes or grapnel-spicules, whose distal ends, bearing the 

 three recurved hooks, frequently project beyond the surface 

 of the sponge. The only thing necessary for the development 

 of an efficient anchoring tuft is the elongation of the shafts of 

 those anatriaenes which project from the lower part of the sponge, 

 and such elongation is exactly what might be expected to take 

 place as the result of fluctuating variation under the influence 

 of natural selection. Exactly the same kind of thing is seen 

 in those curious Trichostemma- and Crinorhiza-forms which 

 support themselves, parachute-like, on soft ooze by throwing 

 out a fringe of long spicules or spicule-bundles, radially arranged, 

 like the ribs of an umbrella. If the intermediate forms happen 

 to disappear, such characters as these may of course be used 

 by the systematist for the discrimination of species just as well 

 as characters that are supposed to arise by mutation. 



In my Presidential Address to the Zoological Section of the 

 British Association for the Advancement of Science, at the 

 Australian meeting in 1914, I had occasion to observe that " it 

 seems likely that mutation has had a great deal to do with the 

 origin of species, though it may have had very little to do with 

 progressive evolution." My friend Dr. Dixie, in a recent article 

 in the Church Quarterly Review, seems to regard this statement as 

 indicative of inconsistency on my part, as I had previously 

 admitted that mutation might be one of the factors of divergent 



