Cyanogenesis-the production of toxic hydrogen cyanide (HCN) by damaged tissue-in Trifolium repens L. (white clover), a type of most important pasture legume, has been studied at different elevations of Darjeeling Himalaya (latitude-27$^{circ}$ 2′ 57′ N, longitude- 88$^{circ}$ 15′ 45" E). Release of HCN takes place due to reaction between cyanogenic glucosides stored in vacuoles of the leaf cell and the corresponding enzyme $eta$-glucosidase present in another compartment, often cell wall. Cyanogenesis, a defense system in plant, protects the clover from herbivore and inhibits grazing. Biochemical analysis showed the presence and absence of the cyanogenesis trait within the population in different proportions at different elevations. Acyanogenic individuals also showed variations with respect to presence or absence of either cyanogenic glucosides or $eta$-glucosidase enzyme or both. The distribution of cyanogenic and acyanogenic plants was found in all places, but at lower altitudes (2084-2094 m) the dominating plants were cyanogenic whereas in higher altitude (2560 m) the dominating plants were acyanogenic. It was observed that blister beetle (Mylabris pustalata Thunb.) and the mollusc (Macrochlamys tusgurium Benson.) were the most common consumer of leaflets of T. repens. Six categories of damage on white clover leaf by these animals were recorded. Our results suggest that the two selective factors or forces i.e. very cold temperature (harmful to cyanogenic plants) at higher altitude as well as indiscriminate but preferential predation (harmful to acyanogenic plants) interact to affect the system of cyanogenesis and also to cause the stable and protective polymorphism in T. repens rather than genotypic differences present among the plants.