THE ESSEX FIELD CLUB.
93
minifera, logarithmic spirals among animals are due to the coiling-up of
more or less evenly-tapering tubes, which is, perhaps, one of the simplest
ways of visualising such spirals.
With regard to plants the logarithmic spiral is very beautifully shown
in the unfolding crozier of a fern frond and in the inflorescence known as
a scorpioid cyme, which is found in the order Boraginaceae, e.g., Forget-
me-not, etc. Spiral lines of an essentially logarithmic character are also .
found in the arrangement of the florets on the head of most composite
flowers, e.g., sun-flower, daisy, etc. The spiral arrangement seen in
the cones of conifers, and even the arrangement of leaves on a plant
(phyllotaxis) have been regarded as in part at least referable to a spiral
of the logarithmic type, although in these cases the matter is complicated
by other factors.
In conclusion it may be said that the occurrence of the logarithmic
spiral in nature seems to point to the existence of what is probably a funda-
mental law of growth, namely, that every increment, during the normal
growing period, has its proportionate effect upon the next increment,
so that healthy growth may be said to follow the rule of compound
interest.2
Following a short discussion a hearty vote of thanks was passed to
the lecturer, and the proceedings terminated.
VISIT TO THE BRITISH MUSEUM (NATURAL HISTORY)
(539th MEETING).
SATURDAY, 10TH DECEMBER, 1921.
A small party of members, 24 in all, met at the Natural History Museum
at 2 o'clock for the purpose of inspecting, by kind invitation of Dr. W. T.
Caiman, F.R.S., F.L.S., the exhibited series of Crustacea under his charge.
Dr. Caiman conducted the party through the Crustacean Gallery,
pointing out many facts of interest in connection with the specimens on
view.
The subjects touched upon included the following:—Range of size
in the Crustacea ; Metamorphosis; Growth; Moulting; Regeneration
of Limbs; Habitats, marine, fresh-water, terrestrial; Plankton forms;
Parasitism and Symbiosis; Fossil Crustacea.
The specimen of the "Coconut Crab" (Birgus latro), exhibited in the
gallery, served as an object lesson for an interesting account of the Land
Crabs, those remarkable decapods which pass their lives on land, climbing
trees, and often roaming to considerable distances from the ocean, into
which, however, they return at the breeding season, to hatch out their
ova ; the young stages are spent in the sea as free-swimming larvae, breath-
ing by means of gills, whereas the adult forms develop, in addition, lungs
adapted for air-breathing, and may be drowned by too-prolonged immer-
sion in the sea.
The President, at the conclusion of Dr. Caiman's lecture, proposed the
thanks of the Club to him for his kindness, and these were heartily accorded
by the members present. Dr. Caiman suitably replied, and the party
then dispersed.
2 The two following books will be found useful by those wishing to study the subject more
deeply:—Growth and Form, by Prof. D'Arcy Thompson (Cambridge, 1917), The Curves of Life,
by Sir T. A. Cook (London, 1914).