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 Calcispongize are remark- 
able for the fluidity and plasticity of their form. Witha 
view to these facts, I made two journeys to the sea-coast 
(1869 to Norway, 1871 to Dalmatia), in order to study as 
