MATERIALS FOR A MONOGRAPH OF THE ASCONS. 567 



system, and for the operation of this principle the spicules 

 offer a splendid field. Few structures are more variable than 

 spicules. In every specimen a certain percentage of irregular, 

 or as we say abnormal spicules are to be found, and sometimes 

 quite a high percentage of them.^ Ascons, again, are for the 

 most part shore forms, and nothing gives a more vivid idea of 

 what the struggle for existence means than, after becoming 

 familiar Avith the fragile and delicate structure of these organ- 

 isms, to watch a stormy sea breaking over the rocks upon 

 which one knows that Ascons are actually living and growing. 

 The wonder is rather that any specimen ever survives. That 

 natural causes produce a high death-rate among these crea- 

 tures can be further deduced from the fact that a single 

 specimen produces an enormous number of larvse, and as the 

 latter exercise but little choice as to the localities in which 

 they fix themselves, a very large proportion must be wiped out 

 of existence by the first storm that breaks over them. All 

 these arguments are perhaps somewhat trite, but they may 

 serve to emphasise the fact that the field is in every way a 

 favorable one for the operation of natural selection. There is 

 variation to supply the material for selection, and there is an 

 environment calculated in every way to favour the survival of 

 individuals distinguished by even slight improvements in the 

 supporting framework. Into the question of the origin of the 

 variations on the one hand, and of how the characters of the 

 fittest, after they have survived, are fixed and become per- 

 petuated on the other hand, this is no place to enter. If, 

 however, the application of the principle of natural selection 

 raises some difficulties in the present state of our knowledge, 

 especially as regards matters of detail, the same is true to a 

 much greater degree of any other theory. Many of the objec- 

 tions so often brought forward against the theory of natural 

 selection seem to me to amount simply to the following argu- 

 ment: there are many facts which natural selection cannot 

 explain, therefore natural selection explains nothing. For my 



* Compare also the facts brought forward above (pp. 521 — 523) with 

 regard to variatiou ia the gastral rays. 



