PEEFACE. XVll 



may draw up, in addition to this, another systematic arrange- 

 ment (more nearly agreeing with the arrangement of the Calci- 

 spongise hitherto in vogue) which gives thirty-nine genera 

 and two hundred and eighty-nine species. A systematist 

 who gives a more limited extension to the " ideal species " 

 might arrange the same series of forms in forty-three genera 

 and three hundred and eighty-one species, or even in one 

 hundred and thirteen genera and five hundred and ninety 

 species ; another systematist, on the other hand, who takes a 

 wider limit for the abstract " species," would use in arrang- 

 ing the same series of forms only three genera, with twenty- 

 one species, or might even satisfy himself with one genus 

 and seven species. The delimitation of species and genera 

 appears to be so arbitrary a matter, on account of endless 

 varieties and transitional forms in this group, that their 

 number is entirely left to the subjective taste of the indi- 

 vidual systematist. In truth, from the point of view of the 

 theory of descent, it appears altogether an unimportant ques- 

 tion as to whether we give a wider or a narrower signifi- 

 cation to allied groups of forms — whether we choose, that is 

 to say, to call them genera or species, varieties or sub-species. 

 The main fact remains undeniable, viz., the common origin 

 of all the species from one ancestral form. The many- 

 shaped Calcareous Sponges furnish, in the very remarkable 

 conditions of their varieties of aggregation (metrocormy), a 

 body of evidence in favour of this view which could hardly 

 be more convincing. Not unfrequently the case occurs of 

 several different forms growing out from a single " stock " 

 or " cormus " — forms which until now have been regarded 

 b}^ systematists, not only as belonging to different species, 

 but even to different genera. Fig. 10 in the frontispiece 



