301 
of the European Ornis, and its Causes. 
ous requirements. The principle of the separation of species is 
in all cases essentially the same ; in respect of its practical ap¬ 
plication, we can only hold different views as to the degree of 
requirement, and this diversity is purely subjective. 
The abrupt gap between the two schools is considerably en¬ 
larged, if we differ also in the principle and not merely in the 
degree of requirement. The dominant zoology universally assumes 
that different species must be actually different in their pro¬ 
perties—that is to say, separated by well-defined limits in their 
characters. This, however, is not theoretically the view of the 
freer unbridled ornithology. In this it stands as a theoretical 
principle that nature everywhere presents transitions between 
species, as between subspecies. With this therefore every kind 
of objective conception falls to the ground. The only means of 
comprehension that remains consists in the comparison of in¬ 
dividuals lying before the observer. The ornithology which pro¬ 
ceeds on the principle of allowing universal transition, and sets 
lax requirements on the species, runs the danger of becoming 
incapable of being checked by others, and thereby necessarily 
renouncing all further influence in favour of a deviation. It is 
throughout of a subjective nature, and therefore not com¬ 
municable. 
From this it follows, in my opinion, that it must remain with¬ 
out results, as, indeed, it has hitherto done, to dispute as to the 
boundaries of species in the two distinct ornithological schools. 
All that can be done towards an elucidation is to indicate the 
irreconcileable degree of the diversity in the two tendencies and 
to establish a synonymy for both views. 
Both the digressing sects do well when they avoid all disputes; 
when they at the utmost confine themselves to ascertaining what 
objective matter of fact runs parallel to the two different sub¬ 
jective views. This is the only question on which unity can be 
attained on both sides, although not to the satisfaction of both 
parties. 
All that the ornithologists with more rigid requirements can 
do in this direction for the advancement of their science is that 
they should come to an understanding among themselves as to 
the way in which the so-called local races, about fifty in number, 
