Note 36 — Chapter II · ON MAGNITUDE

It would seem to be a common if not a general rule that marine organisms, zoophytes, molluscs, etc., tend to be larger than the cor­re­spon­ding and closely related forms living in fresh water. While the phenomenon may have various causes, it has been attributed (among others) to the simple fact that the forces of growth are less antagonised by gravity in the denser medium (cf. Houssay, La Forme et la Vie, 1900, p. 815). The effect of gravity on outward form is illustrated, for instance, by the contrast between the uniformly upward branching of a sea-weed and the drooping curves of a shrub or tree.