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Organs with the same general function combine to form organ systems. For example, the respiratory system includes organs that work together to exchange gas between cells and the atmosphere; organs in the skeletal system support the body and protect the soft internal organs. A suite of organ systems combine to form the human organism. Notice that each layer of complexity involves a group of related units from the preceding layer. This type of hierarchy is found throughout biology and the natural world.

Controlled experimentation. First, we will assess each This silly example shows how scientists may manipperson’s skin tautness by measuring the skin fold that ulate factors that (according to the hypothesis) seem can be pulled up on the back of the hand. Then we related to the observation, all in an attempt to disprove will randomly divide the participants into two groups: the hypothesis. We develop a hypothesis using induca control group and an experimental group. We will tive reasoning—creating a general statement from treat each group in an identical manner, except that our observations.

2. What are some other reasons to value biodiversity besides the ones mentioned? It’s hard to know exactly how far along we are in the current wave of extinction because biologists are not even sure how many species inhabit the Earth. 9 million species have been described, but it is estimated that the total number is several times that. The World Conservation Union reports that 748 species are already extinct, and another 16,119 are threatened with extinction. These threatened organisms include one in three amphibians, one in four coniferous trees and mammals, and one bird in eight.