In the marsupials it is more evident, and its excitation by electric currents evokes movements in the musculature of the crossed side of the body.
If these spark balls are set at the right distance, then when the potential difference accumulates the antenna will be charged and at some stage suddenly discharged by the discharge leaping across the spark gap. This was Marconi's original method, and the plan is still used under the name of the direct method of excitation or the plain antenna.
It is reasonable, therefore, to conclude that ornament is a stimulus to sexual selection, and this conclusion is enforced by the fact that among many comparatively nude peoples clothing is assumed at certain dances which have as their confessed object the excitation of the passions of the opposite sex.
The striking discovery was, in 1903, made by the same investigators that the spontaneous luminosity of radium gives a spectrum of a kind never before obtained without the aid of powerful excitation, electrical or thermal.
Hitherto we were entirely and still are generally confined to electrical excitation or to chemical action as in the case of flames.