(I) Differences in climate and ecology explain the different kinds of organisms found in the past and today. (II) The existence of giant insects and birds suggests that something was very different in the past. (III) Attempts' to link culture with climate and ecology have an old history. (IV) Perhaps the air pressure was higher and some past catastrophe caused much of the earth's atmosphere to be lost to space. (V) Perhaps the composition of the atmosphere was somewhat different