LEARNING BY MARINE INVERTEBRATES
53
marine forms. Young (1958) has shown that octopuses are more liable
to attack objects seen in their aquaria in the evening than in the morning, a finding that correlates with activity records (Denton and GilpinBrown, 1960; Wells, unpublished) which show that at least some cephdopods are more active at night. In Octopus the diurnal fluctuation in
level of attack does not seem to be great enough to effect the results of
discrimination experiments. But this is not the case in some of the
terrestrial and freshwater animals that have been studied (Arbit, 1957 :
earthworms learnmore rapidly in evening trials ; Best, 1965 : planarians
learn best at periods in phase with lunar cycles). Tidal periods might
well be critical in the case of many marine forms.
Tested under appropriate conditions, most animals can be
shown to learn something sometime. I n assessing their capabilities in
this respect it has to be remembered that learning by experience is a
wasteful process and that selection for innate responses is particularly
liable to take place in marine animals, many of which live in restricted
environments that have probably remained practically unchanged
over long periods of time. It seems reasonable to suppose that the
capacity to change with experience is a primitive feature of all but the
simplest nervous systems and that the degree to which learning is
restricted or developed in each species depends on a nice balance
between the advantages of knowing the answers at the outset and the
extravagance of a genetic programme sufficiently elaborate to give a
wide-ranging animal a reasonable chance of survival. Learning may
indeed be a more reliable means of ensuring a high proportion of correct
responses in situations where the programme would be elaborate,
simply because of the uncertainties produced by genetic variation.
No animal can start life without any built-in experience and few live in
sufficiently simple and stable conditions for all responses to be innate.
Somewhere between the two are most marine invertebrates. For each
species there are bound to be areas where individual experience has
been sufficiently constant generation by generation to have favoured
the development of innate responses. It is not suprising that the occurrence of learning is erratic and the absence of the capacity is not
remarkable in any instance unless it can be shown that the survival of
the species suffers in consequence. The idea, that it would be possible
to trace the evolution of learning mechanisms from a survey of invertebrate animals is a sound one only if the series selected is comparable in
respect of the things that each system has to do.
The ideal approach to learning is, therefore, in the context of the
ethology of the animals under study. I n practice, work has usually
started with experiments on learning and bogged down when it became
3.
53
marine forms. Young (1958) has shown that octopuses are more liable
to attack objects seen in their aquaria in the evening than in the morning, a finding that correlates with activity records (Denton and GilpinBrown, 1960; Wells, unpublished) which show that at least some cephdopods are more active at night. In Octopus the diurnal fluctuation in
level of attack does not seem to be great enough to effect the results of
discrimination experiments. But this is not the case in some of the
terrestrial and freshwater animals that have been studied (Arbit, 1957 :
earthworms learnmore rapidly in evening trials ; Best, 1965 : planarians
learn best at periods in phase with lunar cycles). Tidal periods might
well be critical in the case of many marine forms.
Tested under appropriate conditions, most animals can be
shown to learn something sometime. I n assessing their capabilities in
this respect it has to be remembered that learning by experience is a
wasteful process and that selection for innate responses is particularly
liable to take place in marine animals, many of which live in restricted
environments that have probably remained practically unchanged
over long periods of time. It seems reasonable to suppose that the
capacity to change with experience is a primitive feature of all but the
simplest nervous systems and that the degree to which learning is
restricted or developed in each species depends on a nice balance
between the advantages of knowing the answers at the outset and the
extravagance of a genetic programme sufficiently elaborate to give a
wide-ranging animal a reasonable chance of survival. Learning may
indeed be a more reliable means of ensuring a high proportion of correct
responses in situations where the programme would be elaborate,
simply because of the uncertainties produced by genetic variation.
No animal can start life without any built-in experience and few live in
sufficiently simple and stable conditions for all responses to be innate.
Somewhere between the two are most marine invertebrates. For each
species there are bound to be areas where individual experience has
been sufficiently constant generation by generation to have favoured
the development of innate responses. It is not suprising that the occurrence of learning is erratic and the absence of the capacity is not
remarkable in any instance unless it can be shown that the survival of
the species suffers in consequence. The idea, that it would be possible
to trace the evolution of learning mechanisms from a survey of invertebrate animals is a sound one only if the series selected is comparable in
respect of the things that each system has to do.
The ideal approach to learning is, therefore, in the context of the
ethology of the animals under study. I n practice, work has usually
started with experiments on learning and bogged down when it became
3.