A comparison of structural-functional features of genomic DNAs allowed to estimate the role of internal and external factors in evolution of different groups of organisms. The basic difference between higher and lower organisms has been demonstrated. It is reflected in the difference of their reaction on to external factors in accordance with two adaptation types, the openness and autonomization. There is a correlation between structural-functional organization of genomic DNAs of higher and lower organisms and the above mentioned types of adaptation. DNA of lower organisms has been proposed to be characterized as "labile", and that of higher organisms, as "stable". The "DNA lability" means high mutation ability, which characterizes the existence of and evolution of lower organisms (genetic inconstancy of the lower organisms). On the contrary, "DNA stability" means the creation of stable genetic apparatus, reduction of variability in higher organisms (genetic constancy of higher organisms). This suggests the existence of the two principal ways of evolution.