for mating pairs of dogs had likewise created the whole variety of shapes and sizes of these common pets—ranging from a Great Dane to a dachshund.

  1. Animals living in the wild can face a tremendous struggle for survival. For some birds, for example, fewer than one in 100 animals born in one year will survive over a harsh winter into a second year. Those with characteristics best suited for a particular environment—for example, those individual birds who are best able to find scarce food in the winter while avoiding becoming food for a larger animal—tend to have better chances of surviving. Darwin called this process natural selection to distinguish it from the artificial selection used by dog and pigeon breeders to determine which animals to mate to produce offspring.

At least 20 years elapsed between the time that Darwin conceived of descent with modification and 1859, the year that he revealed his ideas to the world in On the Origin of Species. Throughout these 20 years, Darwin did what scientists today do: he tested his ideas of how things work with new observations and experiments. In part, he did this by thinking up every possible objection he could to his own hypothesis.

The ability to analyze individual biological molecules has added great detail to biologists' understanding of the tree of life. For example, molecular analyses indicate that all living things fall into three domains—the Bacteria, Archaea, and Eucarya—related by descent from a common ancestor.

For each such argument, Darwin tried to find an observation made by others, make an observation, or do an experiment of his own that might imply that his ideas were in fact not valid. When he could successfully counter such objections, he strengthened his theory. For example, Darwin's ideas readily explained why distant oceanic islands were generally devoid of terrestrial mammals, except for flying bats. But how could the land snails, so common on such islands, have traversed the hundreds of miles of open ocean that separate the islands from the mainland where the snails first evolved? By floating snails on salt-water for prolonged periods, Darwin convinced himself that, on rare occasions in the past, snails might in fact have "floated in chunks of drifted timber across moderately wide arms of the sea."

This example shows how a hypothesis can drive a scientist to do experiments that would otherwise not be done. Prior to Darwin, the existence of land snails and bats, but not typical terrestrial mammals, on the oceanic islands was simply noted and catalogued as a fact. It is unlikely that anyone would have thought to test the snails for their ability to survive for prolonged periods in salt water. Even if they had, such an experiment would have had little meaning or impact.

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