agenesis_.[4] By this is meant the following phenomenon: Certain
animals, as the salpa and doliolum of the order of the tunicata, as well as
certain mites and many tape-worms, produce offspring which are wholly
dissimilar to the mother stock. These offspring have the capacity of
reproducing themselves--if not by sexual means, as at the first generation,
still by the formation of sprouts; and it is only the animals originated by
the second generation (with many species, even those by the third) which
return again to the form of the first generation. The plant-lice transmit
themselves through six, seven, even ten generations by means of sprouts,
until a generation appears which lays eggs. Now it is indeed true that the
change of generation forms a circle in which the form of the last
generation always returns to that of the first, and therefore leaves the
species, as species, wholly unchanged. But it is nevertheless a process
which shows that the natural law of an identity between generator and
product, observed in other relations, is not without exception; and if we
once have reason to suppose that the generation of new species took place
in past periods of the globe, but has ceased in the present, such processes
in the single period open to our direct observation--namely, the present
(in which, however, according to our knowledge, the species remain
constant)--are {75} nevertheless hints worthy of notice. For they refer us
to ways in which in those former times, when certainly new species did
originate, this formation of species might possibly have taken place.

This consideration leads us to treat of the main objection raised to every
descent theory: namely, that never yet has the origin of one species from
another been observed, but that, on the contrary, _all species_--so far as
our experience goes, stretching over thousands of years--_remain constant_.
We will give no weight to the fact that the constancy of species seems by
no means to be absolutely without exception; for on the whole, they
certainly remain constant. The only example which goes to prove such an
evolution of species as taking place to-day--viz: the natural history of
sponges--seems not to have this bearing. The transitions of form, proven by
O. Schmidt in the siliceous sponges and by Haeckel in the chalk-sponges,
seem to show, not the genetic coming forth of a new species out of another,
and especially not the evolution of a higher species out of a low