XVI PREFACE. 



large a number of individuals as possible in their natural 

 circumstances, and to collect specimens for comparison. Of 

 many species, I 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 limitation of 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 



