  We study persistence of periodic and homoclinic orbits, first integrals and
commutative vector fields in dynamical systems depending on a small parameter
$\varepsilon>0$ and give several necessary conditions for their persistence.
Here we treat homoclinic orbits not only to equilibria but also to periodic
orbits. We also discuss some relationships of these results with the standard
subharmonic and homoclinic Melnikov methods for time-periodic perturbations of
single-degree-of-freedom Hamiltonian systems, and with another version of the
homoclinic Melnikov method for autonomous perturbations of
multi-degree-of-freedom Hamiltonian systems. In particular, we show that a
first integral which converges to the Hamiltonian or another first integral as
the perturbation tends to zero does not exist near the unperturbed periodic or
homoclinic orbits in the perturbed systems if the subharmonic or homoclinic
Melnikov functions are not identically zero on connected open sets. We
illustrate our theory for four examples: The periodically forced Duffing
oscillator, two identical pendula coupled with a harmonic oscillator, a
periodically forced rigid body and a three-mode truncation of a buckled beam.
