258
HILARY B. MOORE
as such and not to either the seasonal temperature variation or to
latitude.
We have no intention of entering here into a discussion of the
various theories on the cause of tropical speciation. Two of the possible
causes are relevant, though. By one theory there are more species, and
therefore more niches in the tropics, and, as a consequence, the niches
are smaller. By the other theory, which we have suggested, the niches
are smaller in the tropics because of stress and the resulting reduced
tolerance ranges. As a consequence there is room for more niches
which, in the course of evolution, have been filled by an increased
number of species. Either sequence might well be true, but the former
would imply stronger pressure of interspecific competition and would,
therefore, be one more example of increased stress in the tropics.
VI. SUMMARY
Data are presented on a number of aspects in which, it is suggested,
stress is less in mid-latitudes and greater in the tropics ; it is usually
greater also in polar seas. In the tropics temperatures are high and
their seasonal range small. Tropical salinities are slightly lower, and
their seasonal range greater than in mid-latitudes. Tropical tidal
ranges are small, but the ratio of spring to neap ranges large.
The range of temperature tolerance is low in the tropics. Individual
species there occupy a smaller fraction of the intertidal zone, and
critical levels are shifted downwards. Tropical growth rates are high
but very variable and there is less growth after sexual maturity than in
mid-latitudes. The seasonal pattern of tropical growth is very variable.
Longevity is less. The proportion of large species is less in the tropics,
although there are some giant species there.
Variability, often associated with stress, is great in the tropics.
For the average species the breeding period is longer and centers later
in the summer than in temperate waters. Adaptation to the tropics
may involve slowed growth, great size and extended vertical intertidal range. Those species which show such adaptation are unable to
live outside the tropics. Taken together, these aspects seem to indicate
that tropical seas are regions of incremed stress.
VII. ACKNOWLEDGMENTS
I am grateful to many people who have helped by providing data
and suggestions. These include the following in addition to many
members of the Institute of Marine Sciences, especially the library
staff: R. T. Abbott, H. D. Albertson, J. D. Andrews, F. 0. Bingham,
HILARY B. MOORE
as such and not to either the seasonal temperature variation or to
latitude.
We have no intention of entering here into a discussion of the
various theories on the cause of tropical speciation. Two of the possible
causes are relevant, though. By one theory there are more species, and
therefore more niches in the tropics, and, as a consequence, the niches
are smaller. By the other theory, which we have suggested, the niches
are smaller in the tropics because of stress and the resulting reduced
tolerance ranges. As a consequence there is room for more niches
which, in the course of evolution, have been filled by an increased
number of species. Either sequence might well be true, but the former
would imply stronger pressure of interspecific competition and would,
therefore, be one more example of increased stress in the tropics.
VI. SUMMARY
Data are presented on a number of aspects in which, it is suggested,
stress is less in mid-latitudes and greater in the tropics ; it is usually
greater also in polar seas. In the tropics temperatures are high and
their seasonal range small. Tropical salinities are slightly lower, and
their seasonal range greater than in mid-latitudes. Tropical tidal
ranges are small, but the ratio of spring to neap ranges large.
The range of temperature tolerance is low in the tropics. Individual
species there occupy a smaller fraction of the intertidal zone, and
critical levels are shifted downwards. Tropical growth rates are high
but very variable and there is less growth after sexual maturity than in
mid-latitudes. The seasonal pattern of tropical growth is very variable.
Longevity is less. The proportion of large species is less in the tropics,
although there are some giant species there.
Variability, often associated with stress, is great in the tropics.
For the average species the breeding period is longer and centers later
in the summer than in temperate waters. Adaptation to the tropics
may involve slowed growth, great size and extended vertical intertidal range. Those species which show such adaptation are unable to
live outside the tropics. Taken together, these aspects seem to indicate
that tropical seas are regions of incremed stress.
VII. ACKNOWLEDGMENTS
I am grateful to many people who have helped by providing data
and suggestions. These include the following in addition to many
members of the Institute of Marine Sciences, especially the library
staff: R. T. Abbott, H. D. Albertson, J. D. Andrews, F. 0. Bingham,