of these
changes would have been no greater than the differences we observe to-day
between the individuals of the very same species, but became in the course
of time so massed and strengthened in one direction that new species have
been evolved. This hypothesis is called the "Theory of Development," {26}
or "Evolution." We retain this name, although well aware of the fact that
the authors do not agree in their use of the term "evolution." Professor
Wigand, who adopts only the theory of a descent from one primordial cell to
another, and who positively rejects the idea of a progress from one fully
developed species to another, claims among other things that one value of
his own theory is that he secures for the idea of evolution its full
meaning. The expression still has a meaning for those who reject the real
descent of the species or their primordial germs one from another, and
acknowledge only the ideal bond of a common plan in their successive
manifestations. But as soon as we examine more closely the literal and
logical meaning of the word, we shall find it of most weight when we
understand by it the before-mentioned gradual evolution in opposition to
the theory of progress by leaps or new creations. Moreover, it is well
known that long before this no other term than evolution was used to
designate the growth of a single organic individual from the primordial
cell and egg to its fully developed form and vital function. Besides, we
find ourselves also in harmony with most of the authors, so far as they
have distinct conceptions of the different scientific problems, if we use
the term "theory of evolution" for the gradual development of one species
from another, in opposition to the hypothesis of a metamorphosis of germs,
or even of a genealogy of primordial cells.

But each evolution theory leads again to new theories, as soon as it has to
be proved in a scientific way. For it can claim a scientific worth only
when sustained by earnest attempts to find and prove the {27} productive
power, agencies and laws of such an evolution of species. Those attempts
can be made in various ways. As a philosophical question, many attempts at
solution have been made, both in ancient and modern times; but being mainly
in the realm of metaphysics, they do not come within the limits of our
scientific essay. As a question for investigators of natural phenomena,
only two attempts of sufficient importance to be mentioned have been made.
The