PREFACE. XV 



and Adaptation in the struggle for existence, also appears to 

 these persons not sufficient. They demand, over and above, 

 that the descent of species from common ancestral forms 

 shall be proved in a particular case ; that, in contradistinc- 

 tion to the synthetic proofs adduced for the Descent Theory, 

 the analytic proof of the genealogical continuity of the 

 several species shall be brought forward. 



This " analytical solution of the problem of the origin of 

 species " I have myself endeavoured to afford in my recently 

 published " Monograph of the Calcareous Sponges." For five 

 consecutive years I have investigated this small but highly 

 instructive group of animals in all its forms in the most 

 careful manner, and I venture to maintain that the mono- 

 graph, which is the result of those studies, is the most 

 complete and accurate morphological analysis of an entire 

 organic group which has up to this time been made. 

 Provided with the whole of the material for study as yet 

 brought together, and assisted by numerous contributions 

 from all parts of the world, I was able to work over the 

 whole group of organic forms known as the Calcareous 

 Sponges in that greatest possible degree of fulness which 

 appeared indispensable for the proof of the common origin 

 of its species. This particular animal group is especially 

 fitted for the analytical solution of the species problem, 

 because it presents exceedingly simple conditions of organ- 

 ization, because in it the morphological conditions possess a 

 greatly superior, and the physiological conditions an inferior, 

 import, and because all species of Calcispongise are remark- 

 able for the fluidity and plasticity of their form. With a 

 view to these facts, I made two journeys to the sea-coast 

 (1869 to Norway, 1871 to Dalmatia), in order to study as 



