Fuerbringer's great work, published in the year 1888 by the _Natura Artis
Magistra_ Society of Amsterdam, enabled Gadow not only to continue for
the next five years the same lines of morphological research, but also
further to investigate those questions which were still left in abeyance
or seemed to require renewed study. The resulting "classification is
based on the examination, mostly autoptic, of a far greater number of
characters than any that had preceded it; moreover, they were chosen in
a different way, discernment being exercised in sifting and weighing
them, so as to determine, so far as possible, the relative value of
each, according as that value may vary in different groups, and not to
produce a mere mechanical 'key' after the fashion become of late years
so common" (Newton's _Dictionary of Birds_, Introduction, p. 103). It is
not the quantity but the quality of the anatomical and bionomic
characters which determines their taxonomic value, and a few fundamental
characters are better indications of the affinities of given groups of
birds than a great number of agreements if these can be shown to be
cases of isomorphism or heterophyletic, convergent analogy. Nature
possesses three great educational or developmental schools--terrestrial,
aquatic and aerial life. Each of these affords animal, vegetable or
mixed diet. Animal diet implies the greatest variety with regard to
locality and the modes of procuring the food. Each of these schools
impresses its pupils, in the case of the birds, with its own stamp, but
there are many combinations, since in the course of phyletic development
many a group of birds has exchanged one school for another. Originally
terrestrial groups have taken to an entirely aquatic life, and _vice
versa_; others, originally endowed with the power of flight, have
become, or are transforming themselves into, absolutely cursorial forms;
some members of one group live entirely on seeds, while others have
become fierce fishers, and so forth. Only by the most careful inquiry
into their history can their relationship or pedigree be unravelled. A
statement may now be given of Gadow's classification of birds, in which
the extinct forms have been intercalated so far as possible. The few
characters assigned to the various groups are sufficiently diagnostic
when taken together, although they are not always those upon which the
classification has been established:--