The lateral vibrations of bars of variable section

ontogeny, and it will be ot great interest in the future to see whether this factor has played any part in the evolution of organisms outside the limits of the Phylum Porifera. \\ e do not propose to speculate upon this subject, but it may be well worth while for botanists and zoologists to keep a look out for possibilities in this direction. No doubt the sponges are especially likely to exhibit cases in which vibrations play an important part, because the water-currents which flow through their canal system with considerable strength must tend to set up such vibrations in any elastic bodies of suitable shape embedded in the almost liquid mesoglcea. But even amongst the sponges, the demonstrable cases, so far as is yet known, are confined to members of the single sub-family Spirastrellinse, belonging either to the genus Latrunculia or to a closely related genus. It is by no means impossible, however, that the form of certain other sponge-spicules may have been influenced by vibrations during development, though we are inclined to think that in most cases the action of surface tension is much more likely to afford a profitable field of investigation. In the case of the oxydiseorhabd, however, it does not seem possible to account for the position of the whorls except by the vibratory theory, and it seems highly probable that the same is true of the Latrunculia discorhabd, although the latter has not been submitted to exact mathematical analysis.