300 Dr. J. H. Blasius on the Diversity in the Estimate 
garded as specifically distinct. In these cases coloration is less 
decisive than the force of the conditions of life and develop¬ 
ment; where the latter are not known, it would be well to de¬ 
pend rather on the form than on the colour. Mergus merganser 
has been met with paired with Anas clangula ; it would be sub¬ 
jective and arbitrary, but possible, and therefore, from the sub¬ 
jective point of view, also justifiable, to regard the two as one 
species. In this case also we should have to depend rather on 
the form than on the isolated objective fact. 
Whoever regards the notion of the species as subjective is 
empirically right; but whoever thereby means to deny to the idea 
of species any objective foundation, is certainly in error. The 
dictum “the species is subjective” has therefore truly no signifi¬ 
cation at all. Any one may with equal right assert the opposite. 
The objective data for the practical application of the idea of 
species lie in the general nature of the animal—in the form, size, 
marking and coloration, in the manifestations of life, in the de¬ 
velopment and reproduction of the animal. We know that not 
a single one of these peculiarities in one and the same species is 
rigidly concluded at a single point,—we know that in all pro¬ 
perties variations may occur in one and the same species. For 
the idea of the species, it is, in fact, requisite that concordance 
should occur within the limits of such vacillations, which cannot 
be established a priori ; but that with respect to all other species, 
a sharply-defined boundary, free from all gradual transitions, 
must occur. When both these objective conditions are fulfilled, 
we are justified in ranging any totality of individuals under the 
same specific idea. 
The objective specific conditions may be fulfilled in very dif¬ 
ferent ways and in very different degrees. Whoever makes very 
small requirements, and judges from an isolated peculiarity 
which has acquired no significance in the totality of the organism 
(separating, for example, the young or the female from the old 
male, the small Sparrow-Hawk from the large one, the brown 
Screech-Owl from the grey, or the grey-capped House-Sparrow 
from the brown-headed one), is of course subjectively justified in 
making species; but his species possess a less degree of objective 
justification than those established in accordance with more rigor- 
