the species, and the possibility of several species descending from a
common ancestor, has been closed to-day by the removal of the sharp
limits that had been set up between species and varieties on the one
hand, and species and genera on the other. I gave an analytic proof of
this in my monograph on the sponges (1872), having made a very close
study of variability in this small but highly instructive group, and
shown the impossibility of making any dogmatic distinction of species.
According as the classifier takes his ideas of genus, species, and
variety in a broader or in a narrower sense, he will find in the small
group of the sponges either one genus with three species, or three
genera with 238 species, or 113 genera with 591 species. Moreover, all
these forms are so connected by intermediate forms that we can
convincingly prove the descent of all the sponges from a common
stem-form, the olynthus.

Here, I think, I have given an analytic solution of the problem of the
origin of species, and so met the demand of certain opponents of
evolution for an actual instance of descent from a stem-form. Those
who are not satisfied with the synthetic proofs of the theory of
evolution which are provided by comparative anatomy, embryology,
paleontology, dysteleology, chorology, and classification, may try to
refute the analytic proof given in my treatise on the sponge, the
outcome of five years of assiduous study. I repeat: It is now
impossible to oppose evolution on the ground that we have no
convincing example of the descent of all the species of a group from a
common ancestor. The monograph on the sponges furnishes such a proof,
and, in my opinion, an indisputable proof. Any man of science who will
follow the protracted steps of my inquiry and test my assertions will
find that in the case of the sponges we can follow the actual
evolution of species in a concrete case. And if this is so, if we can