There are two reasons for the shift in tail length in mice. The first reason could be that the tail length of mice is a recessive trait. In a population where there are also dominant traits, these traits may not show but still be in the population via the heterozygous individuals. The second reason that there may be fewer mice with short tails is that this phenotype causes some survival incompatibility that they may not survive to reproductive years or are not chosen by other mice to cross with. The short tail then does not get passes on.
Moist walls - gases dissolve in the moisture helping them to pass across the gas exchange surface. Permeable walls - allow gases to pass through. Extensive blood supply - ensuring oxygen rich blood is taken away from the lungs and carbon dioxide rich blood is taken to the lungs.
