Mutation as an Evolutionary Factor
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The bodies of the human and other higher animals and plants are all made of microscopic units called “cells” (or materials produced by them). This claim is based on the “cell theory” proposed by M. J. Schleiden and T. Schwann about 150 years ago, and the substance of this claim is now widely accepted as a self-evident “truth” by biologists in general. Most human cells have a diameter of the order of 10–100 μm. This entails that ordinarily about 100 million cells are required to make a piece of tissue 1 cm3 in size, and we can see from this how small the cell is in comparison to the scale of our daily lives (the total number of cells in the human body is about 1013).