Oct. x, 1885.] The Australasian Scientific Magazine. 103 
length of the period, under the common influences to which plants are 
exposed, coincides with the astronomical day. The power of accommoda- 
tion which vegetables possess in this respect is so far from being such as 
either to leave the existence of this periodical constitution doubtful, or to 
entitle us to suppose that the day might be considerably lengthened or 
shortened without injury to the vegetable kingdom. 
Here, then, we have an adaptation between the structure of plants, and 
the periodical order of light and darkness, which arises from the earth’s 
rotation ; and we find, moreover, that the arbitrary quantity in the two 
laws, the length of the cycle of the physiological and of the astronomical 
fact, is the same. Can this have occurred any otherwise than by an inten- 
tional adjustment ? 
Any supposition that the astronomical cycle has occasioned the physio- 
logical one, that the structure of plants has been brought to be what it is by 
the action of external causes, or that such plants as could not accommo- 
date themselves to the existing day, have perished, would be not only an 
arbitrary and baseless assumption, but, moreover, useless for the purposes 
of explanation which it professes, as we have noticed of a similar supposi- 
tion with respect to the annual cycle. 
How came plants to have periodicity at all in those functions which have 
a relation to light and darkness? This part of their constitution was suited 
to organized things, which were to flourish on the earth, and it is accord- 
ingly bestowed on them ; it was necessary for this end that the period 
should be of a certain length ; it is of that length and no other. Surely 
this looks like intentional provision. 
Animals also have a period in their functions and habits; as in the habits 
of waking, sleeping, eating, etc., and their well-being appears to depend on 
the coincidence of this period with the length of the natural day. We see 
that in the day, as it now is, all animals find seasons for taking food and 
repose, which agree perfectly with their health and comfort. Some 
animals feed during the day, as nearly all the ruminating animals and land 
birds ; others feed only in the twilight, as bats and owls, and are called 
crepuscular ; while many beasts of prey, aquatic birds, and others, take 
their food during the night. Those animals which are nocturnal feeders 
are diurnal sleepers, while those which are crepuscular sleep partly in the 
night and partly in the day; but in all, the complete period of these 
functions is twenty-four hours. Man, in like manner, in all nations 
and ages, takes his principal rest once in twenty-four hours; and the regu- 
larity of this practice seems most suitable to his health, though the 
duration of the time allotted to repose is extremely different in different 
eases. So far as we can judge, this period is of a length beneficial to the 
human frame, independently of the effect of external agents. In the 
voyages recently made into high northern latitudes, where the sun did not 
rise for three months, the crews of the ships were made to adhere, with the 
utmost punctuality, to the habit of retiring to rest at nine, and rising a quarter 
before six ; and they enjoyed, under circumstances apparently the most- 
trying, a state of salubrity quite remarkable. This shows, that according 
to the common constitution of such men, the cycle of twenty-four hours 
is very commodious, though not imposed on them by external circum- 
stances. The hours of food and repose are capable of such wide modifica- 
tions in animals, and above all in man, by the influence of external 
stimulants and internal emotions, that it is not easy to distinguish what 
portion of the tendency to such alternations depends on original constitution. 
