  We revisit the origin of Larson's scaling laws describing the structure and
kinematics of molecular clouds. Our analysis is based on recent observational
measurements and data from a suite of six simulations of the interstellar
medium, including effects of self-gravity, turbulence, magnetic field, and
multiphase thermodynamics. Simulations of isothermal supersonic turbulence
reproduce observed slopes in linewidth-size and mass-size relations. Whether or
not self-gravity is included, the linewidth-size relation remains the same. The
mass-size relation, instead, substantially flattens below the sonic scale, as
prestellar cores start to form. Our multiphase models with magnetic field and
domain size 200 pc reproduce both scaling and normalization of the first Larson
law. The simulations support a turbulent interpretation of Larson's relations.
This interpretation implies that: (i) the slopes of linewidth-size and
mass-size correlations are determined by the inertial cascade; (ii) none of the
three Larson laws is fundamental; (iii) instead, if one is known, the other two
follow from scale invariance of the kinetic energy transfer rate. It does not
imply that gravity is dynamically unimportant. The self-similarity of structure
established by the turbulence breaks in star-forming clouds due to the
development of gravitational instability in the vicinity of the sonic scale.
The instability leads to the formation of prestellar cores with the
characteristic mass set by the sonic scale. The high-end slope of the core mass
function predicted by the scaling relations is consistent with the Salpeter
power-law index.
