XVi PREFACE. 
large a number of individuals as possible in their natural 
circumstances, and to collect specimens for comparison. Of 
many species, | compared several hundred individuals in the 
most careful way. I examined with the microscope and 
measured in the most accurate manner the details of form of 
all the species. As the final result of these exhaustive 
and almost endless examinations and measurements it 
appeared that “good species,” in the ordinary dogmatic 
sense of the systematists, have no existence at all among 
the Calcareous Sponges; that the most different forms are 
connected one with another by numberless gradational 
transition forms; and that all the different species of Calca- 
reous Sponges are derived from a single exceedingly simple ~ 
ancestral form, the Olynthus. A drawing of the Olynthus 
and its earliest stages of development (observe especially the 
highly important Gastrula) is given in the frontispiece of 
the present edition. Illustrations of the various structural 
details which establish the derivation of all Calcareous 
Sponges from the Olynthus, are given in the atlas of 
sixty plates which accompanies my monograph of the 
group. In the gastrula, moreover, is now also found the 
common ancestral form from which all the tribes of animals 
(the lowest group, that of the protozoa, alone being excepted) 
can without difficulty be derived. It is one of the most 
ancient and important ancestors of the human race! 
If we take for the limitationof genus and species an average 
standard, derived from the actual practice of systematists, and 
apply this to the whole of the Calcareous Sponges at present 
known, we can distinguish about twenty-one genera, with one 
hundred and eleven species (as I have done in the second 
volume of the Monograph). I have, however, shown that we 
