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Wood first appeared in plants called progymnosperms.
Surgical instruments prepared for a procedure.
Photograph of the flea that transmits Yersinia pestis to cause bubonic plague, called the oriental r
The principle of faunal succession uses animal fossils to tell relative time.
In 2008, the U.S. economy suffered a gaping wound as several trillion dollars were ripped out of it.
A thin cleft, called the synapse, separates one cell from another.