SOME BIOLOGICAL PROBLEMS.
3
tacking on to a specific account of the Entomostraca of one
limited area some observations about the group in general, to
refer to a number of general biological problems and to see to
what extent they can be illustrated by one small group of
animals.
The problems to which I propose to make reference may be
classed roughly as morphological, physiological, ecological
and evolutionary. These categories are not, however, to be
regarded as at all rigidly defined, as they necessarily overlap
in many cases. Some problems, indeed, could equally well be
grouped under more than one, or perhaps under any, of the
heads mentioned.
The first morphological problem to which I would like to call
attention is that of asymmetry. Now, symmetry of some
kind, radial or bilateral, is such a usual feature of the animal
kingdom that any departure from it at once raises interesting
questions of a general character connected with the mechanism
of growth and heredity. We want to know how such asym-
metrical structures are produced, if they can be of any use, and
how they can become fixed.
Among the animals which we are more usually accustomed
to see and hear about any marked departure from the rule of
bilateral symmetry is, of course, very exceptional. The vast
majority of mammals, birds, reptiles, amphibians, fishes, insects
and other arthropods appear, externally at least, to have the
right and left sides of their bodies formed in such a way that
every detail on one side is matched by a corresponding detail on
the other, not indeed by its exact replica, but by its mirror
image. A few striking exceptions to this rule, however, do
occur as, for example, the narwhal, in which the left tusk (canine
tooth) alone is developed; the flat-fishes, in which the right
and left sides (upper and lower surfaces) of the body are very
dissimilar in colouring and the eyes in the adult are both on
one side; and the lobster, in which the two claws are quite
different, one being adapted for crushing and the other for
tearing the prey. Small variations from absolute correspondence
of the two sides, however, are not uncommon, and even where
no difference, or scarcely any, can be detected in size or structure,
differences in function or aptitude may often be noticed, as in
right- and left-handedness in man. Internally the organs of