Symbiosis, first defined by botanist Anton de Bary in 1878, typically refers to a long-term association between two or more species. And the intimacy between the organisms varies depending on the association. Symbiotic associations are varied, and may include, but are not limited to, interactions where one organism benefits but not the other, where neither organism benefits, where both organisms benefit. Intimate partnerships involve a physical fusing together of two or more different organisms, with each providing a survival benefit to the other. This may or may not involve the merging of their genomes, known as symbiogenesis. The first organisms were simple systems: bacteria and viruses. Biologist Lynn Margulis has demonstrated that under extreme environmental stress, one type of bacteria would invade another. Sometimes the invader would fail to kill the host. Likewise the host would be unable to repel the invader. Overnight, rather than gradually, a new kind of relationship was formed; an organism would emerge more complex than either of the two alone, better able to compete, survive, and reproduce. In addition, external relationships have evolved between species, such as flowering plants and insects, or trees and fungi. Flowering plants, which are immobile, and free-flying insects are less intimate partners. They share a deep behavioral connection. For example, flowers attract bees and supply them with nectar. The bees, in turn, brush up against the plant’s sticky pollen spreading it from flower to flower, which insures the reproduction of more plants. Symbiosis is a continually evolving biological process that is responsible for increasingly complex morphology throughout nature.