GROWTH OF IDEAS 271 



and cognate forms, that are often called " sponges " 

 in common parlance. He must think rather 

 of the sponge he uses in his bath. The bath- 

 sponge is a structure made up of very tough, 

 elastic, horny fibres. This structure is originally 

 the skeleton, as it were, of certain animals that 

 are known as " sponge-animals " or, briefly, 

 sponges ; they have nothing to do with the spongy 

 mushrooms I spoke of. At the same time these 

 socially-living sponges are such curious creatures 

 that it was disputed for a long time whether they 

 were real animals or not. There was a second 

 controversy in regard to them as to where the 

 " individual " began what was a single animal, 

 and what a co-operative colony of animals. The 

 latter point alone would have been enough to 

 direct Haeckel's attention to this group after he 

 had, in the case of the siphonophores, gone so 

 deeply into the mystery of combined individuals, 

 forming a new " state-individual." His own 

 opinion eventually was that as a matter of fact in 

 the majority of cases the whole sponge is a stock 

 or colony of separate sponge individuals closely 

 connected together. They had not, indeed, any- 

 thing like the ingenious method of division of 

 labour that we find in the social medusae ; in 

 fact, the sponges are in all respects much more 

 lowly organised animals than the medusae. But 

 they were certainly true animals. And in the 

 middle of his efforts to prove this Haeckel tra- 

 velled into an entirely new field of research, lying 

 far beyond the theory of individuality. 



