_The Length of the Day._
We shall now consider another astronomical element, the time of
the revolution of the earth on its axis; and we shall find here
also that the structure of organized bodies are suited to this
element;--that the cosmical and physiological arrangements are
adapted to each other.
We can very easily conceive the earth to revolve on her axis faster
or slower than she does, and thus the days to be longer or shorter
than they are, without supposing any other change to take place.
There is no apparent reason why this globe should turn on its axis
just three hundred and sixty-six times while it describes its orbit
round the sun. The revolutions of the other planets, so far as
we know them, do not appear to follow any rule by which they are
connected with the distance from the sun. Mercury, Venus, and Mars
have days nearly the length of ours. Jupiter and Saturn revolve
in about ten hours each. For any thing we can discover, the earth
might have revolved in this or any other smaller period; or we
might have had, without mechanical inconvenience, much longer days
than we have.
But the terrestrial day, and consequently the length of the cycle
of light and darkness, being what it is, we find various parts
of the constitution both of animals and vegetables, which have
a periodical character in their functions, corresponding to the
diurnal succession of external conditions; and we find that the
length of the period, as it exists in their constitution, coincides
with the length of the natural day.
The alternation of processes which takes place in plants by day
and by night is less obvious, and less obviously essential to
their well-being, than the annual series of changes. But there are
abundance of facts which serve to show that such an alternation is
part of the vegetable economy.
In the same manner in which Linnæus proposed a Calendar of Flora,
he also proposed a _Dial of Flora_, or Flower-Clock; and this
was to consist, as will readily be supposed, of plants, which
mark certain hours of the day, by opening and shutting their
flowers. Thus the day-lily (_hemerocallis fulva_) opens at five
in the morning; the _leontodon taraxacum_, or common dandelion,
at five or six; the _hieracium latifolium_ (hawkweed), at seven;
the _hieracium pilosella_, at eight; the _calendula arvensis_, or
marigold, at nine; the _mesembryanthemum neapolitanum_, at ten or
eleven; and the closing of these and other flowers in the latter
part of the day offers a similar system of hour marks.
Some of these plants are thus expanded in consequence of the
stimulating action of the light and heat of the day, as appears by
their changing their time, when these influences are changed; but
others appear to be constant to the same hours, and independent
of the impulse of such external circumstances. Other flowers by
their opening and shutting prognosticate the weather. Plants of
the latter kind are called by Linnæus _meteoric_ flowers, as
being regulated by atmospheric causes: those which change their
hour of opening and shutting with the length of the day, he terms
_tropical_; and the hours which they measure are, he observes, like
Turkish hours, of varying length at different seasons. But there
are other plants which he terms _equinoctial_; their vegetable
days, like the days of the equator, being always of equal length;
and these open, and generally close, at a fixed and positive
hour of the day. Such plants clearly prove that the periodical
character, and the period of the motions above described, do not
depend altogether on external circumstances.
Some curious experiments on this subject were made by Decandolle.
He kept certain plants in two cellars, one warmed by a stove
and dark, the other lighted by lamps. On some of the plants the
artificial light appeared to have no influence, (_convolvulus
arvensis_, _convolvulus cneorum_, _silene fruticosa_) and they
still followed the clock hours in their opening and closing. The
night-blowing plants appeared somewhat disturbed, both by perpetual
light and perpetual darkness. In either condition they accelerated
their _going_ so much, that in three days they had gained half a
day, and thus exchanged night for day as their time of opening.
Other flowers _went slower_ in the artificial light (_convolvulus
purpureus_.) In like manner those plants which fold and unfold
their leaves were variously affected by this mode of treatment.
The _oxalis stricta_ and _oxalis incarnata_ kept their habits,
without regarding either artificial light or heat. The _mimosa
leucocephala_ folded and unfolded at the usual times, whether in
light or in darkness, but the folding up was not so complete as
in the open air. The _mimosa pudica_ (sensitive plant,) kept in
darkness during the day time, and illuminated during the night, had
in three days accommodated herself to the artificial state, opening
in the evening, and closing in the morning; restored to the open
air, she recovered her usual habits.
Tropical plants in general, as is remarked by our gardeners, suffer
from the length of our summer daylight; and it has been found
necessary to shade them during a certain part of the day.
It is clear from these facts, that there is a diurnal period
belonging to the constitution of vegetables; though the succession
of functions depends in part on external stimulants, as light and
heat, their periodical character is a result of the structure of
the plant; and this structure is such, that the length of the
period, under the common influences to which plants are exposed,
coincides with the astronomical day. The power of accommodation
which vegetables possess in this respect, is 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 the arbitrary quantity, 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 intentional
adjustment?
Any supposition that the astronomical cycle has occasioned the
physiological 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 accommodate 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 supposition 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 accordingly 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, &c. 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 regularity
of this practice seems most suitable to his health, though the
duration of the time allotted to repose is extremely different
in different cases. 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 circumstances.
The hours of food and repose are capable of such wide modifications
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. Yet no one can doubt that the
inclination to food and sleep is periodical, or can maintain, with
any plausibility, that the period may be lengthened or shortened
without limit. We may be tolerably certain that a constantly
recurring period of forty-eight hours would be too long for one day
of employment and one period of sleep, with our present faculties;
and all, whose bodies and minds are tolerably active, will probably
agree that, independently of habit, a perpetual alternation of
eight hours up and four in bed would employ the human powers less
advantageously and agreeably than an alternation of sixteen and
eight. A creature which could employ the full energies of his body
and mind uninterruptedly for nine months, and then take a single
sleep of three months, would not be a man.
When, therefore, we have subtracted from the daily cycle of the
employments of men and animals, that which is to be set down to
the account of habits acquired, and that which is occasioned by
extraneous causes, there still remains a periodical character;
and a period of a certain length, which coincides with, or at any
rate easily accommodates itself to, the duration of the earth’s
revolution. The physiological analysis of this part of our
constitution is not necessary for our purpose. The succession of
exertion and repose in the muscular system, of excited and dormant
sensibility in the nervous, appear to be fundamentally connected
with the muscular and nervous powers, whatever the nature of these
may be. The necessity of these alternations is one of the measures
of the intensity of those vital energies; and it would seem that
we cannot, without assuming the human powers to be altered,
suppose the intervals of tranquillity which they require to be
much changed. This view agrees with the opinion of some of the
most eminent physiologists. Thus Cabanis[4] notices the periodical
and isochronous character of the desire of sleep, as well as of
other appetites. He states also that sleep is more easy and more
salutary, in proportion as we go to rest and rise every day at the
same hours; and observes that this periodicity seems to have a
reference to the motions of the solar system.
Now how should such a reference be at first established in the
constitution of man, animals, and plants, and transmitted from one
generation of them to another? If we suppose a wise and benevolent
Creator, by whom all the parts of nature were fitted to their uses
and to each other, this is what we might expect and can understand.
On any other supposition such a fact appears altogether incredible
and inconceivable.