The question you pose is a very vexing one and it tore the Theosophical movement apart. Esoterics tells us that Mercury is a principal planet but Mars is not. Also, Mercury is as old as Venus, or was it Vulcan, I can not remember. So, the Leadbeater hypothesis that Mars and Mercury are part of the Earth Chain 7-globes can be discounted. 

The model that says each sequential step forward means that a globe either descends on or ascends a material step, would imply that Globe 4 on the Moon Chain was etheric and not physical. Yet we see a physical remnant of the Moon in this chain. On reflection, I am coming to the opinion that because the etheric and physical subplanes are intimately bound, even though life on the 4th globe of the Moon Chain was physical (49:5-7), it was more focused on the etheric than we are at the moment. That is why you had the death and mayhem that did exist on the Moon Chain at certain times on the 4th Globe.

If you happen to bump into DK, ask him to clarify the situation.