Empire, yet the eastern half of the Empire, which was even more Christian than the west in geographic extent, fervor, penetration and vast numbers continued on for a thousand years afterwards (although Gibbon did not consider the Eastern Empire to be much of a success). As another example, environmental or weather changes affected the east as much as the west, yet the east did not "fall." Theories will sometimes reflect the particular concerns that historians might have on cultural, political, or economic trends in their own times. Gibbon's criticism of Christianity reflects the values of the Enlightenment; his ideas on the decline in martial vigor could have been interpreted by some as a warning to the growing British Empire. In the 19th century, socialist and anti-socialist theorists tended to blame decadence and other political problems. More recently, environmental concerns have become popular, with deforestation and soil erosion proposed as major factors, and destabilizing population decreases due to epidemics such as early cases of bubonic plague and malaria also cited. Global climate changes of 535–536, perhaps caused by the possible eruption of Krakatoa in 535, as mentioned by David Keys and others, is another example. Ideas about transformation without a distinct fall mirror the rise of the postmodern tradition, which rejects periodization concepts (see metanarrative). What is not new are attempts to diagnose Rome's particular problems; already in the early 2nd century, at the height of Roman power, Juvenal in his Satire X criticized the people's obsession with "bread and circuses". One of the primary reasons for the vast number of theories is the notable lack of surviving evidence from the 4th and 5th centuries. For example, there are so few records of an economic nature that it is difficult to arrive at even a generalization of the economic conditions. As a result, historians must use inductive reasoning in addition to available evidence to imagine how things most probably happened or must use evidence from previous and later periods. As in any field where available evidence is sparse, this ability of historians to imagine the 4th and 5th centuries plays as important a part in shaping our understanding as the available evidence, which means that there is room for endless variety in interpretation. The end of the Western Roman Empire traditionally has been seen by historians to mark the end of the Ancient Era and beginning of the Middle Ages. More