reptiles, amphibians, fishes, plus some small marine animals in the phylum Chordata. Today, many systematists group organisms according to a system known as cladistics. By determining which traits of a species evolved earlier and which evolved later, this system seeks to classify organisms according to their evolutionary history.

Science as Explanation

In the quest for understanding, science involves a great deal of careful observation that eventually produces an elaborate written description of the natural world. This description is communicated to scientists in scientific journals or at scientific meetings, so that others can build on pre-existing work. In this way, the accuracy and sophistication of the description tends to increase with time, as subsequent generations of scientists correct and extend the observations

Evolutionary relationships often are depicted in diagrams that resemble the branches of trees. Closely related species (denoted S1, S2, etc.) are grouped into genera, genera into families, and so on. The result is a hierarchical diagram showing how different species evolved from common ancestor species (represented in this diagram by the letters A through E).

of their predecessors. Because the total sum of scientific knowledge increases relentlessly, scientific progress is something that all scientists take for granted.

But science is not just description. Even as observations are being made, the human mind attempts to sort, or organize, the observations in a way that reveals some underlying order in the objects or phenomena being observed. This sorting process, which involves a great deal of trial and error, seems to be driven by a fundamental human urge to make sense of our world.

The sorting process also suggests new observations that might otherwise not be made. For example, the suggestion that bats should be grouped with mammals led to an intensified examination of the similarities between bats and rodents—first at the anatomical level, and later with respect to the genes and protein molecules that form their cells. In this case, new evidence was obtained that confirmed the suggested relationship. In other cases, the further observations inspired by a tentative grouping have caused the rejection of a new idea.

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