Billions years ago, the earth was a barren land; however,there are flourish grass and flowers and also various beasts and birds with colorful lather on it. How did these happen? Darwin explained that any component part of living things existsand evolves because it is useful to the living thing's survival.

For example, fragrancy and colorfulness of flowers exist forattracting insects to broadcast its nectar; sweetness and beautyof fruits exist for attracting beasts and birds to transfer their seeds around?[1]. The colorful appearances of birds are kept by the nature because they are helpful to win the love of the opposite sex to win victor in sex selection [1]. However, why do insets, birds and beasts have the sense organs that produce the feelings of fragrancy, sweetness and beauty? Darwin wrote(translated from Chinese [1]):

"How did the most simple aesthetic feeling, i.e. the pleasant feeling caused by colours, sounds and shapes, initially develop in the minds of human beings and animals? This is indeed a problem hard to understand. If we ask why some dour and taste can cause pleasant feelings and others cause unpleasant feelings,this is also difficult. In all of these cases, habits must be relative more or less; but there must exist essential reasons in the nervous structure of each species."

It is not difficult to find after serious observation that the objectives that we desire (i.e. want to eat, to approach and so on) generally have good properties such as fragrancy,sweetness and beauty besides the properties that can satisfy our material requirements; vice verse. For example, apples, peanuts,meat and so on are not only nutritious but also taste good;comfortable and pragmatical clothes or environment are generally good?looking; clever, kind, young and healthy person generally has beautiful or handsome appearance. On the contrary, garbage heap, poison caterpillar, red eye, and mildewy peanut are not only harmful to health but also unpleasant, i.e. stinking,bitter, or ugly. If one asks why we like to eat apples, one answer is that they are nutritious; the other answer is that they are fragrant, sweet, and good?tasting. Why do they taste good? It is because they have fragrant and sweet properties. According to this view, the causality resulting in desires is shown as follows:?

However, the above causality clearly has three defects. the first is that two pairs of causes and effects are parallel and without inertial relation; we cannot understand why they happen to always combine together with few exceptions. The second is that the causality is inconsistent with the nature selection theory since according to this theory it is impossible that a living thing has the mechanism of pleasant feelings that exist only for obeying some properties of external objects rather than for benefiting his own existence. The third is that this causality cannot explain the inconsistency between the feelings of different people and animals and the instability of feelings of the same person to the same food. For example, tigers and monkeys have different tastes to apples; spoiled food tastes different to a man when he is an emperor and a beggar.

After the considerations of the above several aspects, I think the following hypothesis is reasonable: historically, the causality from fragrancy, sweetness, beauty to desires should be thoroughly reversed. That means that mankind and some animals like to eat fruits and meats not because fruits and meats tasted good first but because they desired to eat fruits and meats for existence first. We feel people with some appearances beautiful and like to approach them not because these appearances looked beautiful first but because we wanted to approach them (for mutual help or sex selection) first. Similarly, man avoids touching some objects not because they were stinking, bitter or ugly first but because he was necessary to or always wanted to avoid them first. The causality of desires and pleasant feelings is shown in Figure 1. According to this model, the pleasant feelings are feedback signals exciting desires. And the initial causes of their developments are material requirements and desires, or material dissatisfactions; pleasant feelings reflect the requirement relations between men or animals and the external world and reversely improve the developments of these relations at the same time.

Figure 1 the causality determining the evolution of desires and pleasant feelings

The above model can ensures that (1)the phenomenon that two pairs of causes and effects are parallel does not exist; (2)it conforms with the theory of natural selection to hold that

pleasant feelings are feedback signals exciting desires; (3)the changes and developments of pleasant and unpleasant feelings can be explained with the changes and developments of requirement relations. According this model, some objects give us pleasant feelings because they are useful and often dissatisfy with our requirements.

3 The Identity between the New hypothesis and Experiential Facts?

In the following, we use facts to explain whether pleasant feelings reflect and reversely excite requirement relations.1) What generally satisfy with men's requirements can cause pleasant feelings; vice verse.?

2) Feelings are different with different requirement relations. For example, To tigers, only meat tastes good whereas fruits taste terribly; yet, to monkeys, these are opposite.?

3) Pleasant feelings change with requirement relations changing. For example, mankind was being evolved from apes; we can imagine that mankind did not perceive pleasant feelings in eating meat at first; with environment changing and men's hunting ability strengthened, men began to eat meats for energy. Only Later, men perceived pleasant feelings in eating meats. Tea did not give us pleasant feelings in the early; later, because men's food composition and working environment were changed, it was helpful to digestion and refreshment to drink tea properly.

Hence, men forwardly drank tea again and again so that the tea tasted good. Beer and hot peppers are similar. Generally, men and women who have appearances that look young, healthy, noble or kind are handsome or beautiful because they often satisfy the requirements of mutual help or sex selection. If a lady who has the above appearance, yet, has bad moral character or serious infectious disease at the same time, then his appearance will gradually become from the beautiful into the ugly from the view of people who understand her.

4) The intensity of pleasant feeling always changes with the urgency of requirements. For example, bread taste especially good to hungry people; cold water is better than juice to thirsty people; campfire is especially beautiful from the view of a wanderer in cold night; wilderness is especially beautiful from the view of a prisoner in hell.

5) lasting and repeated requirements can have corresponding requirements highly developed. For example, fisherman can appreciate most the beauty of a lotus pool surrounded by weeping willows; hunters particularly like the odors of campfire smoke and gunpowder [2]. These phenomena are generally thought because of habits. However, if we only see habits without seeing requirements, we cannot explain why some frequent actions can easily be habitual whereas some others cannot. Someone might give the following opposite samples:

"Chinese herbs conforms with patients' requirements, but they usually do not give them pleasant feelings." My answer is that the mechanism of feelings is relatively stable as well as social consciousness; these herbs taste bitter because from historical standpoint they are generally not what people require. A consistent fact with the above viewpoint is that if someone subjectively takes some herb again and again for treatment, then this herb probably taste good as well as Chinese prickly ash to Sichuan people ( since it has function of expelling cold and damp).

"Smoke, alcohol, and opium are harmful to man's body, but they can give men intense pleasant feelings." The answer is that Smoke, alcohol, and opium are harmful to man's body; but on the other hand, they have refreshing or similar function, only over?taking them can cause more harm than interest. These is only because the histories of their being taken are shorter so that men's mechanism of pleasant feelings is not adaptable to them. It is not difficult to imagine that those people who enjoy smoke, alcohol, and drugs very much without control will be eliminated through natural selection, and only the mechanism that is adaptable to the new requirement relation can be kept and developed. Feelings and requirements are always adaptable, at the same time not adaptable, and eventually adaptable to each other as well as superstructure and economicbase. The both sides get developed in contradiction and motion.

Based on the above discussion, it is not difficult to understand the evolutions of flowers, fruits, and birds' colorful leathers.

Considering flowers and insects, we can imagine that in the earlier times all flowers in the world were as monotonous as anemophilous flowers such as those of cogongrass, and insects had no sense organs for fragrant and sweet feelings. Since allogamy is especially beneficial to survival, those plants with nectar satisfying insect's requirement were kept more because of the insects' pollination. Meanwhile, the insects that had the function of pleasant feelings corresponding the nectar sought the flowers with stronger motivation and hence took more food so that they were kept more. Since flowers with particular colors and odors were more easy to be found, they got highly developed; meanwhile, the functions of pleasant feelings of the insects also got evolved.

Similarly, in the earlier times there were no fruit with hard core in meat?like cover in the world and there were neither bird nor beast that had the functions of fragrant and sweet feelings corresponding to fruits. Because the fruits with particular colors and nutrition were more easier to be swallowed and broadcast anywhere through excretion by birds and beasts so that the more of them were kept by nature. The birds and beasts with the functions of pleasant feelings also got evolved with their seeking fruits so that fruits and some birds and beasts exist as we see now.?

Some people think that some flowers and fruits look beautiful because they have bright colors and particular shapes. It should be minded that many poisonous snakes, fogs, and insects even many plants' thorns also have multicolored or red appearances; head?red flies also have brightcolored appearance; yet, they are very ugly. Darwin attributed all decorative appearances of animals to the sake of beauty [3]. However, why do the animals have those functions of pleasant feelings? It was unknowable to him. From author's viewpoint, birds and beasts have multicolored appearances because these appearances can?

(1)conceal them conveniently, such as the appearances of hares and frogs;

(2)make the same kind find them conveniently, such as those of pandas and magpies;

(3)identify sex better, such as male lions' beards and cocks' red crowns;

(4)be identifications for warning enemies( an animal with that appearance seemly say "I am not one to be trifled" or "I do not taste good"), such as those of poisonous snakes and head- red flies.

These functions are all related to information transmission. If those appearances can cause beauty feelings of the same kind, the corresponding function of beauty feelings must have got developed in the process of their seeking each other.

However, peacocks and some other birds are exception. The ball?like rounds on a peacock's leather seemly come from purposive creation. The peacock is not alone. A pheasant kind illustrated in Darwin's works Men's origin and Sex Selection also has ball?like round on leathers. Those "balls" seem to be stereoscopic and to be lighted up by the Sun [3]. We have to confess that they have the appearances for the sake of beauty. However, if we investigate the food composition and living environment of these birds, we can easily understand this phenomenon. So these birds most like berries such as grapes [4]. It is not difficult to imagine that the requirement between peacocks and berries had brought the beauty feeling function of peacocks about so that the male peacocks that happened to have ball?like rounds on leathers because of variation could more easily win the females' approaching and hence had more later generations. Whether or not is a male peacock that is spreading and shaking the wings very similar to a tree with fruit hanging heavy? Whether or not are the sharp leathers on a cock's waist somewhat similar to the ears of grain? Whether or not is the pleasing and variable sound of larks similar to the sound of a small stream? As well as mankind views what he lack and hence like as beautiful things and displays them in arts, the birds also view what they lack and hence like as beautiful things and display them on leathers or in "song".

It is the essential principle of historical materialism to explain the developments and changes of literature, art and other superstructures with the developments and changes of material requirement [5]. Seemly this principle can also be extended to biological field. This extension is necessary not only for explaining biological phenomena but also for completing and developing historical materialism itself.?