n theory, and
opposed it by sharp criticisms.

This _migration_ or _isolation theory_ also found a degree of favor, but
subordinate in its nature. For it {54} can not and will not pretend to
solve the main problems. It only tries to explain how the individual
variations, already in existence, might have been preserved and perhaps
increased, and how new conditions of existence could have roused latent
powers; but not how these variations and these powers originated. Just as
little is the selection theory able to explain this; but it pretends to do
it, and hence we can easily comprehend how during the last few years a
constantly increasing number of voices, and more important ones, have been
raised against the selection theory. This opposition came not only from
those who--like Agassiz, Barrande, Emil Blanchard, Escher von der Linth,
Goeppert, Giebel, Sir Roderick Murchison, Pfaff, and others--directly reject
each and every idea of descent on account of the difficulty in defending
the selection theory; or who--like Karl Ernst von Baer,[3] (the {55}
pioneer in the region of the history of individual development), like Oskar
Fraas, Griesebach, Sandberger, and others--generally take a more reserved
and neutral position, because of the uncertainty of the facts and the
inaccessibility of the problems; but it comes especially from those
scientists who are inclined to adopt an origin of species through descent
and even through development, yet refuse to explain it by the selection
principle, and look for the essential cause of the development in the
organisms themselves, without claiming to have themselves found these
causes. Among the most prominent advocates of this view, we may name the
late Sir Charles Lyell, Mivart, and {56} Richard Owen, in England; and in
Germany, Alexander Braun, Ecker, Gegenbaur, Oswald Heer, W. His, Naegeli,
Ruetimeyer, Schaaffhausen, Virchow, Karl Vogt, A. W. Volkmann, Weismann,
Zittel, and here also Moriz Wagner, and among the philosophers, Eduard von
Hartmann. Many of these men are but little aware of the difference between
the two questions: whether, on the one hand, the adoption of the origin of
species through descent does not of itself involve the idea of a gradual
development of one species from another, almost unobservable in its single
steps; or, on the other hand, whether a descent of species through
heterogenetic generation in leaps and through a metamorphosis of the germs,
could be imagined