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use certain diseases。 For example； alterations in the efficiency of the cerebral link may yield neurasthenia； mania； dementia； of the thyroid link； Graves' disease； myxedema； of the adrenal link； Addison's disease； cardiovascular disease。
This introduction may serve to give the line of our argument。 We shall now consider briefly certain salient facts which relate to the conversion of latent into kinetic energy as an adaptive reaction。 The experimental data are so many that they will later be published in a monograph。
The amount of latent energy which may be converted into kinetic energy for adaptive ends varies in different species； in individuals of the same species； in the same individual in different seasons； in the life cycle of growth； reproduction and decay； in the waking and sleeping hours； in disease and in activity。 We shall here consider briefly the reasons for some of those variations and the mechanisms which make them possible。
Biologic Consideration of the Adaptive Variation in Amounts of Energy Stored in Various Animals
Energy is appropriated from the physical forces of nature that constitute the environment。 This energy is stored in the body in quantities in excess of the needs of the moment。 In some animals this excess storage is greater than in other animals。 Those animals whose self…preservation is dependent on purely mechanical or chemical means of defensesuch animals as crustaceans； porcupines； skunks or cobrashave a relatively small amount of convertible （adaptive） energy stored in their bodies。 On the contrary； the more an animal is dependent on its muscular activity for self…preservation； the more surplus available （adaptive） energy there is stored in its body。 It may be true that all animals have approximately an equal amount per kilo of chemical energy but certainly they have not an equal amount stored in a form which is available for immediate conversion for adaptive ends。 Adaptive Variation in the Rate of Energy Discharge
What chance for survival would a skunk have without odor； a cobra without venom； a turtle without carapace； or a porcupine shorn of its barbs； in an environment of powerful and hostile carnivora？ And yet in such an hostile environment many unprotected animals survive by their muscular power of flight alone。 It is evident that the provision for the storage of 〃adaptive〃 energy is not the only evolved characteristic which relates to the energy of the body。 The more the self…preservation of the animal depends on motor activity； the greater is the range of variation in the rate of discharge of energy。 The rate of energy discharge is especially high in animals evolved along the line of hunter and hunted； such as the carnivora and the herbivora of the great plains。
Influences That Cause Variation in the Rate of Output of Energy in the Individual
Not only is there a variation in the rate of output of energy among various species of animals； but one finds also variations in the rate of output of energy among individuals of the same species。 If our thesis that men and animals are mechanisms responding to environmental stimuli be correct； and further； if the speed of energy output be due to changes in the activating organs as a result of adaptive stimulation； then we should expect to find physical changes in the activating glands during the cycles of increased activation。 What are the facts？ We know that most animals have breeding seasons evolved as adaptations to the food supply and weather。 Hence there is in most animals a mating season in advance of the season of maximum food supply so that the young may appear at the period when food is most abundant。 In the springtime most birds and mammals mate； and in the springtime at least one of the great activating glands is enlargedthe thyroid in man and in animals shows seasonal enlargement。 The effect of the increased activity is seen in the song； the courting； the fighting； in the quickened pulse； and in a slightly raised temperature。 Even more activation than that connected with the season is seen in the physical state of mating； when the thyroid is known to enlarge materially； though this increased activity； as we shall show later； is probably no greater than the increased activity of other activating glands。 In the mating season the kinetic activity is speeded up； in short； there exists a statea fleeting stateof mild Graves' disease。 In the early stages of Graves' disease； before the destructive phenomena are felt； the kinetic speed is high； and life is on a sensuous edge。 Not only is there a seasonal rhythm to the rate of flow of energy； but there is a diurnal variationthe ebb is at night； and the full tide in the daytime。 This observation is verified by the experiments which show that certain organs in the kinetic chain are histologically exhausted； the depleted cells being for the most part restored by sleep。
We have seen that there are variations in speed in different species； and that in the same species speed varies with the season of the year and with the time of day。 In addition there are variations also in the rate of discharge of energy in the various cycles of the life of the individual。 The young are evolved at high speed for growth； so that as soon as possible they may attain to their own power of self…defense； they must adapt themselves to innumerable bacteria； to food； and to all the elements in their external environment。 Against their gross enemies the young are measurably protected by their parents； but the parentsexcept to a limited extent in the case of manare unable to assist in the protection of the young against infectious disease。
The cycle of greatest kinetic energy for physiologic ends is the period of reproduction。 In the female especially there is a cycle of increased activity just prior to her development into the procreative state。 During this time secondary sexual characters are developed the pelvis expands； the ovaries and the uterus grow rapidly； the mammary glands develop。 Again in this period of increasing speed in the expenditure of energy we find the thyroid； the adrenals； and the hypophysis also in rapid growth。 Without the normal development of the ovary； the thyroid； and the hypophysis； neither the male nor the female can develop the secondary sexual characters； nor do they develop sexual desire nor show seasonal cycles of activity； nor can they procreate。 The secondary sexual characterssexual desire； fertilitymay be developed at will； for example； by feeding thyroid products from alien species to the individual deprived of the thyroid。
At the close of the child…bearing period there is a permanent diminution of the speed of energy discharge； for energy is no longer needed as it was for the self…preservation of the offspring before adolescence； and for the propagation of the species during the procreative period。 Unless other factors intervene； this reduction in speed is progressive until senescent death。 The diminished size of the thyroid of the aged bears testimony to the part the activating organs bear in the general decline。
We have now referred to variations in the rate of discharge of energy in different species； in individuals of the same species； in cycles in the same individualsuch as the seasons of food supply； the periods of wakefulness and of sleep； the procreative period； and we have spoken of those variations caused artificially by thyroid feeding； thus far having confined our discussion to the conversion for adaptive purposes of latent into kinetic energy in muscular and in procreative action。 We shall now consider the conversion of latent into kinetic energy in the production of heat；＇*＇ and endeavor to answer the questions which arise at once： Is there one mechanism for the conversion of latent energy into heat and another mechanism for its conversion into muscular action？ What is the adaptive advantage of fever in infection？
＇*＇ We use the terms 〃heat〃 and 〃muscular action〃 in the popular sense； though physicists use them to designate one and the same kind of energy。
The Purpose and the Mechanism of Heat Production in Infections
Vaughan has shown that the presence in the body of any alien protein causes an increased production of heat； and that there is no difference between the production of fever by foreign proteins and by infections。 Before the day of the hypodermic needle and of experimental medicine； the foreign proteins found in the body outside the alimentary tract were brought in by invading microorganisms。 Such organisms interfered with and destroyed the host。 The body； therefore； was forced to evolve a means of protection against these hostile organisms。 The increased metabolism and fever in infection might operate as a protection in two waysthe increased fever； by interfering with bacterial growth； and the increased metabolism； by breaking up the bacteria。 Bacteriologists have taught us that bacteria grow best at the normal temperature of the body； hence fever must interfere with bacterial growth。 With each rise of one degree centigrade the chemical activity of the body is increased 10 per cent。 In acute infections there is aversion to food and frequently there is vomiting。 In fever； then； we have diminished intake of e