  We look into the 2851 $^{12}$CO molecular clouds harboring $^{13}$CO
structures to reveal the distribution of the projected angular separations and
radial velocity separations between their internal $^{13}$CO structures. The
projected angular separations are determined using the minimal spanning tree
algorithm. We find that $\sim$ 50$\%$ of the angular separations fall in a
narrow range of $\sim$ 3 - 7 arcmin with a median of $\sim$ 5 arcmin, and the
corresponding radial velocity separations mainly range from $\sim$ 0.3 km
s$^{-1}$ to 2.5 km s$^{-1}$. The mean and standard deviation of the angular
separations of the internal $^{13}$CO structures within $^{12}$CO clouds appear
to be universal, independent of the $^{12}$CO cloud angular areas and the
counts of their internal $^{13}$CO structures. We also reveal a scaling
relation between the $^{12}$CO cloud angular area and its harbored $^{13}$CO
structure count. These results suggest there is a preferred angular separation
between $^{13}$CO structures in these $^{12}$CO clouds, considering the
distance effects. According to that, we propose an alternative picture for the
assembly and destruction of molecular clouds: there is a fundamental separation
for the internal structures of molecular clouds, the build-up and destruction
of molecular clouds proceeds under this fundamental unit.
