102 Mutations and Evolution. 



The views here set forth regarding the occurrence of both 

 mutational and adaptational characters in organisms are entirely 

 in harmony with Dendy's (1913, 1916) conclusions from 

 investigations of the sponges. He concludes that the specific 

 characters of sponges are generally non-adaptive, and that a great 

 many of the spicule differences so characteristic of the group must 

 have originated by mutations. He also points out what we would 

 now regard as parallel mutations, as in the appearance of 

 trichodragmata, and in the polyphyletic family Epipolasidae, which 

 is an assemblage of stelletid sponges which have independently 

 lost their triaeue spicules. Usually the most minute differences, 

 both in megascleres and microscleres, remain constant throughout 

 the species, though millions of spicules may occur in each 

 individual, to whom these microscopic differences cannot be of the 

 slightest use. Such differences point to chemical or physical 

 alterations iu the constitution of the germplasm. 



On the other hand, adaptive modifications of spicules for 

 various purposes have taken place, as in the modification 

 of triaenes into grapnels for anchoring the sponge to a muddy 

 bottom, or into discotriaenes which serve as a dermal protection. 

 Some of these adaptive conditions may have come about through 

 selection among a series of mutations, but it is difficult to see that 

 any of them involve the principle of inheritance of acquired 

 characters. 



