ON NOCTURNAL ANIMALS. 315 
of night; while others, again, roam at early dawn, when stillness 
and imperfect light prevail. ‘The term “diurnal,” in strictness, 
is applicable to the first mentioned; the three others may there- 
fore be classed under the head “nocturnal,” although the term 
“‘erepuscular” is not unfrequently employed to designate those 
animals whose habit it is to issue forth in the glimmering 
betwixt light and darkness. ; 
But the limits of separation are not easily defined, notwith- 
standing breadth employed. Take mankind collectively, and 
without hesitation they would be classed as diurnal creatures ; 
yet a moment’s reflection will show that, either from choice or 
necessity, many individuals lead a truly nocturnal life. Nor can 
it be said the crepuscular element is here wanting, albeit the 
use of artificial light. Again, Herbivores, the Deer tribe par- 
ticularly, are diurnal, yet under certain conditions—i. e., during 
the breeding season—they temporarily become night-roamers. 
Other somewhat corresponding instances of change from ordinary 
habit may more fitly be referred to in the sequel. 
An explanation of some of the peculiar phenomena observable 
among Nocturnal Birds and Mammalia (Bats to wit) would be 
incomplete without a due consideration of the senses and sensory 
organs of a few of the inferior orders of the Invertebrata, as 
elucidating fundamental principles. 
Quite at the bottom of the scale which zoologists have to deal 
with are the group of Protozoa,—creatures all more or less of 
the simplest construction, and for the most part microscopic in 
size. Little indeed is there to be shown in the organization 
of such a form as Protameba primitiva, Haeck, where alone a 
faint greyish coloured and translucent, granular, gelatinous sub- — 
stance is recognizable. This jelly-like material, notwithstanding, 
possesses all the essentials of animals far higher in the scale, 
inasmuch as, without any organs whatsoever, the necessary func- 
tions of organic life and reproduction are performed. Its sarcode 
is sensitive to impressions—viz. subject to irritability; hence 
contraction and expansion—i.e., movement. It suffers waste of 
tissue, hunger follows, and supply means assimilation of nutri- 
ment at any point, and simple division means reproduction. But 
as bearing on our question at issue, of greater signification is 
the remarkable property of this jelly’s having a diffused sense, 
equivalent to touch. Thus this so-called sixth sense of some 
