408 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
emerged more recently, and in several distinct groups. Because of 
their different evolutionary histories, each of these groups of animals 
has a different set of potentialities for life on land. We shall try to 
assess the significance of these differences. 
THE PROBLEMS OF LAND LIFE 
What are the problems concerned in the evolution of a land fauna? 
The comparatively rare medium, air, affords no support for the body, 
and calls for a complete reorganization of the means of locomotion. 
Gills, which are very efficient organs of respiration in water, collapse 
in air and present too small a surface for the uptake of oxygen, so 
that respiratory organs have to be remodeled. The respiratory mem- 
branes themselves, however, must be kept moist and thin, because 
oxygen does not diffuse rapidly enough through a dry integument. 
Changes in metabolic machinery, too, are necessary. Thus, the waste 
products of nitrogen metabolism can most economically be excreted 
as ammonia, and aquatic animals use this method, for although am- 
monia is highly toxic, plenty of water is available to flush it away. 
On dry land, water is not so plentiful, and ammonia cannot be used. 
The simple process of discharging large numbers of reproductive 
cells into the environment for external fertilization must be replaced 
by internal fertilization, which in turn demands complicated behavior 
patterns to insure association of the sexes. The eggs of land animals, 
once fertilized, cannot be discharged and left to fend for themselves. 
They would dry up. Therefore they must be enclosed in impermeable 
shells and provided with food and with metabolic machinery for con- 
verting ammonia into nontoxic and preferably insoluble subtances 
which can be stored during the period of incubation. 
The fact that air is hardly ever saturated with water vapor, and 
is often rather dry, means that a terrestrial animal is in danger of 
losing water continuously by evaporation. 
Finally, the terrestrial environment is one in which changes in 
temperature and humidity occur over a much wider range and more 
rapidly than they do in water. These changes must be tolerated, 
controlled, or avoided. 
Let us now consider how these problems have been solved by 
different kinds of animals. 
RESPIRATION 
The vertebrates solved the problem of respiration in two stages— 
first the amphibians developed an internal lung, which was rather 
inefficient and had to be supplemented by cutaneous respiration. 
This involved a moist skin. Then the reptiles perfected the lung 
and consequently could afford to develop an impermeable integument. 
Respiration in land arthropods is carried out in a number of dif- 
ferent ways. Insects, perhaps the most successful group, have 
