SMALL RANGE OF TEMPERATURE REQUIRED FOR GROWTH AND DEVELOPMENT

Vital phenomena for the most part occur between the temperatures of freezing water and 104 Fahr., and this is supposed to be due mainly to the properties of nitrogen and its compounds, which between these temperatures only can maintain those peculiarities which are essential to life--extreme sensitiveness and lability; facility of change as regards chemical combination and energy; and other properties which alone render nutrition, growth, and continual repair possible. A very small increase or decrease of temperature beyond these limits, if continued for any considerable time, would certainly destroy most existing forms of life, and would not improbably render any further development of life impossible except in some of its lowest forms.

As one example of the direct effects of increased temperature, we may adduce the coagulation of alb.u.men. This substance is one of the proteids, and plays an important part in the vital phenomena of both plants and animals, and its fluidity and power of easy combination and change of form are lost by any degree of coagulation which takes place at about 160 Fahr.

The extreme importance to all the higher organisms of a moderate temperature is strikingly shown by the complex and successful arrangements for maintaining a uniform degree of heat in the interior of the body. The normal blood-heat in a man is 98 Fahr., and this is constantly maintained within one or two degrees though the external temperature may be more than fifty degrees below the freezing-point. High temperatures upon the earth's surface do not range so far from the mean as do the low. In the greater part of the tropics the air-temperature seldom reaches 96 Fahr., though in arid districts and deserts, which occur chiefly along the margins of the northern and southern tropics, it not unfrequently surpa.s.ses 110 Fahr., and even occasionally rises to 115 or 120 in Australia and Central India.

Yet with suitable food and moderate care the blood-temperature of a healthy man would not rise or fall more than one or at most two degrees. The great importance of this uniformity of temperature in all the vital organs is distinctly shown by the fact that when, during fevers, the temperature of the patient rises six degrees above the normal amount, his condition is critical, while an increase of seven or eight degrees is an almost certain indication of a fatal result. Even in the vegetable kingdom seeds will not germinate under a temperature of four or five degrees above the freezing-point.

Now this extreme sensibility to variations of internal temperature is quite intelligible when we consider the complexity and instability of protoplasm, and of all the proteids in the living organism, and how important it is that the processes of nutrition and growth, involving constant motion of fluids and incessant molecular decompositions and recombinations, should be effected with the greatest regularity. And though a few of the higher animals, including man, are so perfectly organised that they can adapt or protect themselves so as to be able to live under very extreme conditions as regards temperature, yet this is not the case with the great majority, or with the lower types, as evidenced by the almost complete absence of reptiles from the arctic regions.

It must also be remembered that extreme cold and extreme heat are nowhere perpetual. There is always some diversity of seasons, and there is no land animal which pa.s.ses its whole life where the temperature never rises above the freezing point.

THE NECESSITY OF SOLAR LIGHT

Whether the higher animals and man could have been developed upon the earth without solar light, even if all the other essential conditions were present, is doubtful. That, however, is not the point I am at present considering, but one that is much more fundamental. Without plant life land animals at all events could never have come into existence, because they have not the power of making protoplasm out of inorganic matter. The plant alone can take the carbon out of the small proportion of carbonic acid in the atmosphere, and with it, and the other necessary elements, as already described, build up those wonderful carbon compounds which are the very foundation of animal life. But it does this solely by the agency of solar light, and even uses a special portion of that light. Not only, therefore, is a sun needed to give light and heat, but it is quite possible that _any_ sun would not answer the purpose. A sun is required whose light possesses those special rays which are effective for this operation, and as we know that the stars differ greatly in their spectra, and therefore in the nature of their light, all might not be able to effect this great transformation, which is one of the very first steps in rendering animal life possible on our earth, and therefore probably on all earths.

WATER A FIRST ESSENTIAL OF ORGANIC LIFE

It is hardly necessary to point out the absolute necessity of water, since it actually const.i.tutes a very large proportion of the material of every living organism, and about three-fourths of our own bodies. Water, therefore, must be present everywhere, in one form or another, on any globe where life is possible. Neither animal nor plant can exist without it. It must also be present in such quant.i.ty and so distributed as to be constantly available on every part of a globe where life is to be maintained; and it is equally necessary that it should have persisted in equal profusion throughout those enormous geological epochs during which life has been developing. We shall see later on how very special are the conditions that have secured this continuous distribution of water on our earth, and we shall also learn that this large amount of water, its wide distribution, and its arrangement with regard to the land-surface, is an essential factor in producing that limited range of temperature which, as we have seen, is a primary condition for the development and maintenance of life.

THE ATMOSPHERE MUST BE OF SUFFICIENT DENSITY AND COMPOSED OF SUITABLE GASES

The atmosphere of any planet on which life can be developed must have several qualities which are unconnected with each other, and the coincidence of which may be a rare phenomenon in the universe. The first of these is a sufficient density, which is required for two purposes--as a storer of heat, and in order to supply the oxygen, carbonic acid, and aqueous vapour in sufficient quant.i.ties for the requirements of vegetable and animal life.

As a reservoir of heat and a regulator of temperature, a rather dense atmosphere is a first necessity, in co-operation with the large quant.i.ty and wide distribution of water referred to in the last section. The very different character of our south-west from our north-east winds is a good ill.u.s.tration of its power of distributing heat and moisture. This it does owing to the peculiar property it possesses of allowing the sun's rays to pa.s.s freely through it to the earth which it warms, but acting like a blanket in preventing the rapid escape of the non-luminous heat so produced. But the heat stored up during the day is given out at night, and thus secures a uniformity of temperature which would not otherwise exist.

This effect is strikingly seen at high alt.i.tudes, where the temperature becomes lower and lower, till at a not very great elevation, even in the tropics, snow lies on the ground all the year round. This is almost wholly due to the rarity of the air, which, on that account, has not so much capacity for heat. It also allows the heat it acquires to radiate more freely than denser air, so that the nights are much colder. At about 18,000 feet high our atmosphere is exactly half its density at the sea-level. This is considerably higher than the usual snow-line, even under the equator, whence it follows that if our atmosphere was only half its present density it would render the earth unsuitable for the higher forms of animal life.

It is not easy to say exactly what would be the result as regards climate; but it seems likely that, except perhaps in limited areas in the tropics, where conditions were very favourable, the whole land-surface would become buried in snow and ice. This appears inevitable, because evaporation from the oceans by direct sun-heat would be more rapid than now; but as the vapour rose in the rare atmosphere it would rapidly become frozen, and snow would fall almost perpetually, although it might not lie permanently on the ground in the equatorial lowlands. It appears certain, therefore, that with half our present bulk of atmosphere life would be hardly possible on the earth on account of lowered temperature alone. And as there would certainly be an added difficulty in the needful supply of oxygen to animals and carbonic acid to plants, it seems highly probable that a reduction of density of even one-fourth might be sufficient to render a large portion of the globe a snow and ice-clad waste, and the remainder liable to such extremes of climate that only low forms of life could have arisen and been permanently maintained.

THE GASES OF THE ATMOSPHERE

Coming now to consider the const.i.tuent gases of the atmosphere, there is reason to believe that they form a mixture as nicely balanced in regard to animal and vegetable life as are the density and the temperature. At a first view of the subject we might conclude that oxygen is the one great essential for animal life, and that all else is of little importance. But further consideration shows us that nitrogen, although merely a diluent of the oxygen as regards the respiration of animals, is of the first importance to plants, which obtain it from the ammonia formed in the atmosphere and carried down into the soil by the rain. Although there is only one part of ammonia to a million of air, yet upon this minute proportion the very existence of the animal world depends, because neither animals nor plants can a.s.similate the free nitrogen of the air into their tissues.

Another fundamentally important gas in the atmosphere is carbonic acid, which forms about four parts in ten thousand parts of air, and, as already stated, is the source from which plants build up the great bulk of their tissues, as well as those protoplasms and proteids so absolutely necessary as food for animals. An important fact to notice here is, that carbonic acid, so essential to plants, and to animals through plants, is yet a poison to animals. When present in much more than the normal quant.i.ty, as it often is in cities and in badly ventilated buildings, it becomes highly prejudicial to health; but this is believed to be partly due to the various corporeal emanations and other impurities a.s.sociated with it. Pure carbonic acid gas to the amount of even one per cent. in otherwise pure air can, it is said, be breathed for a time without bad effects, but anything more than this proportion will soon produce suffocation. It is probable, therefore, that a very much smaller proportion than one per cent., if constantly present, would be dangerous to life; though no doubt, if this had always been the proportion, life might have been developed in adaptation to it.

Considering, however, that this poisonous gas is largely given out by the higher animals as a product of respiration, it would evidently be dangerous to the permanence of life if the quant.i.ty forming a constant const.i.tuent of the atmosphere were much greater than it is.

AQUEOUS VAPOUR IN THE ATMOSPHERE

This water-gas, although it occurs in the atmosphere in largely varying quant.i.ties, is yet, in two distinct ways, essential to organic life. It prevents the too rapid loss of moisture from the leaves of plants when exposed to the sun, and it is also absorbed by the upper surface of the leaf and by the young shoots, which thus obtain both water and minute quant.i.ties of ammonia when the supply by the roots is insufficient. But it is of even more vital importance in supplying the hydrogen which, when united with the nitrogen of the atmosphere by electrical discharges, produces the ammonia, which is the main source of all the proteids of the plant, which proteids are the very foundation of animal life.

From this brief statement of the purposes served by the various gases forming our atmosphere, we see that they are to some extent antagonistic, and that any considerable increase of one or the other would lead to results that might be injurious either directly or in their ultimate results. And as the elements which const.i.tute the bulk of all living matter possess properties which render them alone suitable for the purpose, we may conclude that the proportions in which they exist in our atmosphere cannot be very widely departed from wherever organic forms are developed.

THE ALTERNATION OF DAY AND NIGHT

Although it is difficult to decide positively whether alternations of light and darkness at short intervals are absolutely essential for the development of the various higher forms of life, or whether a world in which light was constant might do as well, yet on the whole it seems probable that day and night are really important factors. All nature is full of rhythmic movements of infinitely varied kinds, degrees, and durations. All the motions and functions of living things are periodic; growth and repair, a.s.similation and waste, go on alternately. All our organs are subject to fatigue and require rest. All kinds of stimulus must be of short duration or injurious results follow. Hence the advantage of darkness, when the stimuli of light and heat are partially removed, and we welcome 'tired nature's sweet restorer, balmy sleep'--giving rest to all the senses and faculties of body and mind, and endowing us with renewed vigour for another period of activity and enjoyment of life.

Plants as well as animals are invigorated by this nightly repose; and all alike benefit by these longer periods of greater and less amounts of work caused by summer and winter, dry and wet seasons. It is a suggestive fact, that where the influence of heat and light is greatest--within the tropics--the days and nights are of equal length, giving equal periods of activity and rest. But in cold and Arctic regions where, during the short summer, light is nearly perpetual, and all the functions of life, in vegetation especially, go on with extreme rapidity, this is followed by the long rest of winter, with its short days and greatly lengthened periods of darkness.

Of course, all this is rather suggestion than proof. It is possible that in a world of perpetual day or in one of perpetual night, life _might_ have been developed. But on the other hand, considering the great variety of physical conditions which are seen to be necessary for the development and preservation of life in its endless varieties, any prejudicial influences, however slight, might turn the scale, and prevent that harmonious and continuous evolution which we know _must_ have occurred.

So far I have only considered the question of day and night as regards the presence or absence of light. But it is probably far more important in its heat aspect; and here its period becomes of great, perhaps vital, importance. With its present duration of twelve hours day and twelve night on the average, there is not time, even between the tropics, for the earth to become so excessively heated as to be inimical to life; while a considerable portion of the heat, stored up in the soil, the water, and the atmosphere, is given out at night, and thus prevents a too sudden and injurious contrast of heat and cold. If the day and night were each very much longer--say 50 or 100 hours--it is quite certain that, during a day of that duration, the heat would become so great as to be inimical, perhaps prohibitive, to most forms of life; while the absence of all sun-heat for an equally long period would result in a temperature far below the freezing point of water. It is doubtful whether any high forms of animal life could have arisen under such great and continual contrasts of temperature.

We will now proceed to point out the special features which, in our earth, have combined to bring about and to maintain the various and complex conditions we have seen to be essential for life as it exists around us.

CHAPTER XII

THE EARTH IN ITS RELATION TO THE DEVELOPMENT AND MAINTENANCE OF LIFE

The first circ.u.mstance to be considered in relation to the habitability of a planet is its distance from the sun. We know that the heating power of the sun upon our earth is ample for the development of life in an almost infinite variety of forms; and we have a large amount of evidence to show that, were it not for the equalising power of air and water, distributed as they are with us, the heat received from the sun would be sometimes too great and sometimes too little. In some parts of Africa, Australia, and India, the sandy soil becomes so hot that an egg can be cooked by placing it just below the surface. On the other hand, at an elevation of about 12,000 feet in lat. 40 it freezes every night, and throughout the day in all places sheltered from the sun. Now, both these temperatures are adverse to life, and if either of them persisted over a considerable portion of the earth, the development of life would have been impossible. But the heat derived from the sun is inversely as the square of the distance, so that at half the distance we should have four times as much heat, and at twice the distance only one-fourth of the heat. Even at two-thirds of the distance we should receive more than twice as much heat; and, considering the facts as to the extreme sensitiveness of protoplasm and the coagulation of alb.u.men, it seems certain that we are situated in what has been termed the temperate zone of the solar system, and that we could not be removed far from our present position without endangering a considerable portion of the life now existing upon the earth, and in all probability rendering the actual development of life, through all its phases and gradations, impossible.

THE OBLIQUITY OF THE ECLIPTIC

The effect of the obliquity of the earth's equator to its path round the sun, upon which depend our varying seasons and the inequality of day and night throughout all the temperate zones, is very generally known. But it is not usually considered that this obliquity is of any great importance as regards the suitability of the earth for the development and maintenance of life; and it seems to have been pa.s.sed over as an accident hardly worth notice, because almost any other obliquity or none at all would have been equally advantageous. But if we consider what the direction of the earth's axis might possibly have been, we shall find that it is really a matter of great importance from our present point of view.

Let us suppose, first, that the earth's axis was, like that of Ura.n.u.s, almost exactly in the plane of its...o...b..t or directed towards the sun. There can be little doubt that such a position would have rendered our world unfitted for the development of life. For the result would be the most tremendous contrasts of the seasons; at mid-winter, on one half the globe, arctic night and more than arctic cold would prevail; while on the other half there would be a midsummer of continuous day with a vertical sun and such an amount of heat as nowhere exists with us. At the two equinoxes the whole globe would enjoy equal day and night, all our present tropics and part of the sub-tropical zone having the sun at noon so near to the zenith as to have the essential of a tropical climate. But the change to about a month of constant suns.h.i.+ne or a month of continuous night would be so rapid, that it seems almost impossible that either vegetable or animal life would ever have developed under such terrible conditions.

The other extreme direction of the earth's axis, exactly at right angles to the plane of the orbit, would be much more favourable, but would still have its disadvantages. The whole surface from equator to poles would enjoy equal day and night, and every part would receive the same amount of sun-heat all the year round, so that there would be no change of seasons; but the heat received would vary with the lat.i.tude. In our lat.i.tude the sun's alt.i.tude at noon all the year would be less than 40, the same as now occurs at the equinoxes, and we might therefore have a perpetual spring as regards temperature. But the constancy of the heat in the equatorial and tropical regions and of cold towards the poles would lead to a more constant and more rapid circulation of air, and we should probably experience such continuous north-westerly winds as to render our climate always cold and probably very damp. Near the poles the sun would always be on, or close to, the horizon, and would give so little heat that the sea might be perpetually frozen and the land deeply snow-buried; and these conditions would probably extend into the temperate zone, and possibly so far south as to render life impossible in our lat.i.tudes, since whatever results arose would be due to permanent causes, and we know how powerful are snow and ice to extend their sway over adjacent areas if not counteracted by summer heat or warm moist winds. On the whole, therefore, it seems probable that this position of the earth's axis would result in a much smaller portion of its surface being capable of supporting a luxuriant and varied vegetable and animal life than is now the case; while the extreme uniformity of conditions everywhere present might be so antagonistic to the great law of rhythm that seems to pervade the universe, and be in other ways so unfavourable, that life-development would probably have taken quite a different course from that which it has taken.

It appears almost certain, therefore, that some intermediate position of the axis would be the most favourable; and that which actually exists seems to combine the advantage of change of seasons with good climatical conditions over the largest possible area. We know that during the greater part of the epoch of life-development this area was much greater than at present, since a luxuriant vegetation of deciduous and evergreen trees and shrubs extended up to and within the Arctic Circle, leading to the formation of coal-beds both in palaeozoic and tertiary times; the extremely favourable conditions for organic life which then prevailed over so large a portion of the globe's surface, and which persisted down to a comparatively recent epoch, lead to the conclusion that no more favourable degree of obliquity was possible than that which we actually possess. A short account of the evidence on this interesting subject will now be given.

PERSISTENCE OF MILD CLIMATES THROUGH GEOLOGIC TIME

The whole of the geological evidence goes to show that in remote ages the climate of the earth was generally more uniform, though perhaps not warmer, than it is now, and this can be best explained by a slightly different distribution of sea and land, which allowed the warm waters of the tropical oceans to penetrate into various parts of the continents (which were then more broken up than they are now), and also to extend more freely into the Arctic regions. So soon as we go back into the tertiary period, we find indications of a warmer climate in the north temperate zone; and when we reach the middle of that period, we find abundant indications, both in plant and animal remains, of mild climates near to the Arctic Circle, or actually within it.

On the west coast of Greenland, in 70 N. lat., there are found abundance of fossil plants very beautifully preserved, among which are many different species of oaks, beeches, poplars, plane-trees, vines, walnuts, plums, chestnuts, sequoias, and numerous shrubs--137 species in all, indicating a vegetation such as now grows in the north temperate parts of America and Eastern Asia. And even further north, in Spitzbergen, in N. lat. 78 and 79, a somewhat similar flora is found, not quite so varied, but with oaks, poplars, birches, planes, limes, hazels, pines, and many aquatic plants such as may now be found in West Norway and in Alaska, nearly twenty degrees further south.

Still more remote, in the Cretaceous period, fossil plants have been found in Greenland, consisting of ferns, cycads, conifers, and such trees and shrubs as poplars, sa.s.safras, andromedas, magnolias, myrtles, and many others, similar in character and often identical in species with fossils of the same period found in Central Europe and the United States, indicating a widespread uniformity of climate, such as would be brought about by the great ocean-currents carrying the warm waters of the tropics into the Arctic seas.

Still further back, in the Jura.s.sic period, we have proofs of a mild climate in East Siberia and at Ando in Norway just within the Arctic Circle, in numerous plant remains, and also remains of great reptiles allied to those found in the same strata in all parts of the world. Similar phenomena occur in the still earlier Tria.s.sic period; but we will pa.s.s on to the much more remote Carboniferous period, during which most of the great coal-beds of the world were formed from a luxuriant vegetation, consisting mostly of ferns, giant horse-tails, and primitive conifers. The luxuriance of these plants, which are often found beautifully preserved and in immense quant.i.ties, is supposed to indicate an atmosphere in which carbonic acid gas was much more abundant than now; and this is rendered probable by the small number and low type of terrestrial animals, consisting of a few insects and amphibia.

But the interesting point is, that true coal-beds, with similar fossils to those of our own coal-measures, are found at Spitzbergen and at Bear Island in East Siberia, both far within the Arctic Circle, again indicating a great uniformity of climate, and probably a denser and more vapour-laden atmosphere, which would act as a blanket over the earth and preserve the heat brought to the Arctic seas by the ocean currents from the warmer regions.

The still earlier Silurian rocks are also found abundantly in the Arctic regions, but their fossils are entirely of marine animals. Yet they show the same phenomena as regards climate, since the corals and cephalopodous mollusca found in the Arctic beds closely resemble those of all other parts of the earth.[16]

Many other facts indicate that throughout the enormous periods required for the development of the varied forms of life upon the earth, the great phenomena of nature were but little different from those that prevail in our own times. The slow and gentle processes by which the various vegetable and animal remains were preserved are shown by the perfect state in which many of the fossils exist. Often trunks of trees, cycads, and tree-ferns are found standing erect, with their roots still imbedded in the soil they grew in. Large leaves of poplars, maples, oaks, and other trees are often preserved in as perfect a state as if gathered by a botanist and dried between paper for his herbarium, and the same is especially the case with the beautiful ferns of the Permian and Carboniferous periods. Throughout these and most other formations well-preserved ripple-marks are found in the solidified mud or sand of old seash.o.r.es, differing in no respect from similar marks to be found on almost every coast to-day. Equally interesting are the marks of rain-drops preserved in the rocks of almost all ages. Sir Charles Lyell has given ill.u.s.trations of recent impressions of rain-drops on the extensive mud-flats of Nova Scotia, and also an ill.u.s.tration of rain-drops on a slab of shale from the carboniferous formation of the same country; and the two are as much alike as the prints of two different showers a few days apart. The general size and form of the drops are almost identical, and imply a great similarity in the general atmospheric conditions.

We must not forget that this presence of rain throughout geological time implies, as we have seen in our last chapter, a constant and universal distribution of atmospheric dust. The two chief sources of this dust--the total quant.i.ty of which in the atmosphere must be enormous--are volcanoes and deserts, and we are therefore sure that these two great natural phenomena have always been present. Of volcanoes we have ample independent evidence in the presence of lavas and volcanic ashes, as well as actual stumps or cores of old volcanoes, through all geological formations; and we can have little doubt that deserts also were present, though perhaps not always so extensive as they are now. It is a very suggestive fact that these two phenomena, usually held to be blots on the fair face of nature, and even to be opposed to belief in a beneficent Creator, should now be proved to be really essential to the earth's habitability.