As the Civil War raged, William Brewer, a young botanist from upstate New York, spent five years cataloging California’s natural attributes for its Legislature. As he and his crew traversed the state by mule in their annual sojourns, living off the land, Brewer found much to commend. But in letters to his brother, decades later assembled into a must-read book, “Up and Down California,” Brewer also wondered whether its climate would impede its development.