physiological value. In respect to this we must especially insist that
the question of hybrid offspring, the last corner of refuge of all the
defenders of the constancy of species, has at present lost all
significance as bearing on the conception of species. For we know now,
through numerous and reliable experiences and experiments, that two
different true varieties can frequently unite and produce fertile
hybrids (as the hare and rabbit, lion and tiger, many different kinds
of the carp and trout tribes, of willows, brambles, and others); and
in the second place, the fact is equally certain that descendants of
one and the same species which, according to the dogma of the old
schools, could always effect a fertile union under certain
circumstances, either cannot effect such a union or produce only
barren hybrids (the Porto-Santo rabbit, the different races of horses,
dogs, roses, hyacinths, &c.; see "History of Creation," vol. i., p.
146).

For a certain proof that the conception of species rests on a
subjective abstraction and has a merely relative value--like the
conception of genus, family, order, class, &c.--no class of animals is
of so much importance as that of the Sponges. In it the fluctuating
forms vary with such unexampled indefiniteness and variability as to
make all distinction of species quite illusory. Oscar Schmidt has
already pointed this out in the siliceous sponges and keratose
sponges; and I, in my monograph, in three volumes, on the Calcareous
Sponges (the result of five years of most accurate investigations of
this small animal group), have pointed out that we may at pleasure
distinguish 3, or 21, or 111, or 289, or 591 different species. I also
believe that I have thus convincingly demonstrated how all these
different forms of the calcareous sponges may quite naturally, and
without any forcing, be traced to a single common parent-form, the
simple--and not hypothetical, but existing at this present day--the
simple Olynthus. Hence I think I have here produced the most positive
analytical evidence of the transformation of species, and of the unity
of the derivation of all the species of a given group of animals, that
is generally possible.

Properly, I might spare myself these disquisitions on the question of
species, for Virchow does not go into this main question of the theory
of descent--but this is very characteristic of his attitude. And just
as he nowhere thoroughly discusses the doctrine of tr