PREFACE. Xvi 
may draw up, in addition to this, another systematic arrange- 
ment (more nearlyagreeing with the arrangement of the Calci- 
spongiz hitherto in vogue) which gives thirty-nine genera 
and two hundred and eighty-nime 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 
by systematists, not only as belonging to different species, 
but even to different genera. Fig. 10 in the frontispiece 
