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 surpasses 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 passes its whole life where the temperature never rises above the freezing point. THE NECESSITY OF SOLAR LIGHT