THE president's ADDRESS. ' 43 



ance of destructive ratlier than to the disappearance of construc- 

 tive agents. 



We find then, in sponges as in higher groups of animals, 

 many characters that are obviously adaptive and many others 

 that appear to have no such significance, and characters of 

 both categories are used by systematists as indices of genetic 

 affinity, for the purpose of constructing a natural, phylogenetic 

 arrangement of the organisms in question. 



As to which set of characters is to be regarded as the more 

 important from the point of view of the student of progressive 

 evolution there can, I think, be little doubt, but how far the 

 division into adaptive and non-adaptive corresponds to the dis- 

 tinction between fluctuating variation and mutation is a different 

 question. Certainly the chances are greatly against a mutation, 

 when it first appears, having any adaptive significance. The 

 apical ray of a calcareous quadriradiate may perhaps arise as a 

 mutation, at any rate there seems to be no mechanical necessity 

 for its appearance. It may remain quite short and without 

 obvious significance for the well-being of the sponge, as in the 

 dermal quadriradiates of some species of Leucetta, but, in other 

 cases, it may be seized upon," so to speak, as a new weapon in 

 the struggle for existence and used as one of the most important 

 constituents of the skeleton, as in the Amphoriscidae. Many of 

 the specific characters of the microscleres, again, may have arisen 

 as mutations, but they seem to have remained entirely meaning- 

 less from the point of view of function. 



It seems to me incredible that the long series of adaptive changes 

 which have taken place in the evolution, say, of the skeleton of 

 the higher Calcarea, and which are so closely correlated with 

 one another and with the evolution of the canal-system, can 

 have been entirely or even mainly dependent upon the accidental 

 occurrence of mutations, which are supposed to take place in 

 all directions and without any relation to the necessities of the 

 organism. When, for example, the dermal cortex appeared 

 no doubt scleroblasts wandered into it as is their wont, and there 

 gave rise to triradiate spicule-systems tangentially arranged. 

 The whole process is obviously determined chiefly by mechanical 

 conditions. The establishment of one modification gives oppor- 



