Cells are the basic living units of all contemporary organisms with the exception of viruses. In some species of organisms, the body consists of a single cell (single-cell organisms); in other species, i.e. multi-cellular organisms, the cells multiply repeatedly during ontogeny, are variously relocated and diversify until, compared with the microscopic cells, they form the enormous bodies of a multi-cellular organism. The human body is apparently formed of about 50 trillion (i.e. 5 × 1013) cells.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more