THE CALCAREOUS SPONGES. I4I 



" good species " of which, in fact, cannot be spoken of in the 

 usual systematic sense. We find among them only varying 

 series of forms, which do not even completely transmit their 

 specific form to their nearest descendants, but by adaptation 

 to subordinate, external conditions of existence, perpetually 

 change. It frequently occurs here, that there arise out of 

 one and the same stock different form-species, which accord- 

 ing to the usual system would belong to several quite distinct 

 genera ; this is the case, for instance, with the remarkable 

 Ascometra (Frontispiece, Fig. 10.) The entire external bodily 

 form is much more pliable and protean in Calcareous Sponges 

 than in the silicious sponges, which are characterized by 

 possessing silicious spicula, forming a beautiful skeleton. 

 Through the study of the comparative anatomy and ontogeny 

 of calcareous sponges, we can recognise, with the greatest 

 certainty, the common primary form of the whole group, 

 namely, the sack-shaped Olynthus, whose development is 

 represented in the Frontispiece (compare its explanation in 

 the Appendix). Out of the Olynthus (Fig. 9 on the Frontis- 

 piece), the order of the Ascones was the first to develop, out 

 of which, at a later period, the two other orders of Cal- 

 careous Sponges, the Leucones and Sy cones, arose as diverg- 

 ing branches. Within these orders, the descent of the 

 individual forms can again be followed step by step. Thus 

 the Calcareous Sponges in every respect confirm the pro- 

 })osition which I have elsewhere maintained : that " the 

 natural history of sponges forms a connected and striking 

 argument in favour of Darwin." 



The second main class or branch in the tribe of Zoophytes 

 is formed by the Sea-nettles (Acalephse, or Cnidso). This 

 interesting group of animals, so rich in forms, is composed 



