The net system both deciding reciprocity among over hundred characteristics and forming mechanism of seed yield performance had been determined through research in thirty one years (Figure 1). This was basic principle of engineering of crop breeding [1] [2] .

Firstly, analyzing and researching basic contradiction of yield performance of a variety, the expression formula was raised. Yield performance was decided by contradiction between adaptability and yield ability, environment playing role through the contradiction [1] (Figure 2).

The yield ability (or yield potential) of a variety yielded under the most suitable condition for the variety. But in practice, a variety often had been planted in relatively unfavorable environment, and it had adaptability for the environment. The formula was important advance for formula of “Phenotype = Genotype + Environment” that had been continued in more than one hundred and fifty years, and it revealed internal contradiction of yield performance of a genotype. The formula of basic contradiction laid groundwork to differentiate the two different types of characteristics and set up the net system including all of ecological traits and yield ability traits. This was also a reason of no setting up the basic principle of crop breeding, even though crop breeding had already history in more than one hundred and fifty years.

Secondly, nature, action and significance of ecological traits were determined. Comparison tests using of similar isolines of glowing period and pod bearing habit in soybean in multiple years discovered that varieties having suitable ecotype could fuller display its yield ability and gave better yield stability in much years [3] [4] , and unsuitable ecology characteristics (such as excessively early or late mature period) had obviously negative influence on else ecological traits and all of yield ability traits. After this, leaf size [5] , resistance to lodging [6] , root system type [7] , resistance to diseases and pests, all kinds of physiological resistance [2] [8] etc. were determined to belong to ecology-adaptability characteristic.

Thirdly, all of yield ability traits and reciprocal relationships among these yield ability traits were determined. They were mainly based on research accomplishments of crop physiology [9] , using axiom method [10] to set up complete theory system of yield ability and net system of trait components [2] .

It was obvious that if there only was test, no support of advanced philosophy thinking and scientific theory thought and scientific method theory, the basic principle of crop breeding could not be completed.

Basic principle of crop breeding was first time discovered and revealed in the paper in English. Study on net system of characteristics genetics of yield display of a variety was a great theory problem that was whole or overall in crop studies. Evolvement trend of crop cultivar in future can be scientifically foreseen by this study achievement. Development of studies on breeding and cultivar, characteristic inheritance will be greatly promoted. This net system was also the basis of studies on molecule genetics. The basic principle was a milestone transforming experience breeding into scientific breeding of crops, also a transition milestone from Mendel’s genetics that could only explain quality characteristics inheritance into both quality and quantity, and was a great breakthrough of studies on genetics and breeding of crops.

Ecology breeding was the breeding improving ecology characteristics and enhancing yield stability of variety. Research inclusions were mainly producing conditions and essence, classification, inheritance and variation, adaptive form and adaptive area division, breeding of all ecological characteristics, study on enhancing extensive adaptability of varieties.

Yield ability breeding was the breeding improving yield ability characteristics and enhancing yield ability (or potential) of varieties. Main study inclusions were improving seed yield per plant and its component parts, economic coefficient (transformation ability), biological yield per plant and its composition parts, plant shape etc.,

and comprehensive breeding study of yield ability traits, analyzing and studying basic regularity of yield ability breeding.

Quality breeding belongs to a part of seed yield breeding (sink) in yield ability breeding. Seed yield itself existed in contradiction and contradiction movement of “quality” and “quantity”.