422 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
presented all at once, rather than one at a time as they are along the 
other route. Now the littoral habitat is particularly subject to violent 
change, and a high transpiration rate would be of value both in 
permitting oxygen uptake in dry air through the integument, and in 
enabling the animals to reach shelter if they were caught in the open 
by intense solar radiation. This may be an instance of a group of 
animals having been caught in an evolutionary cul-de-sac, because 
the immediate survival value of a highly permeable cuticle prevented 
the development of those characters essential to land life. At least 
it can be said with confidence that until the twin problems of an 
efficient internal respiratory system and a greater temperature tol- 
erance have been solved, the integument cannot be made imperme- 
able and the animals will be prevented from full exploitation of the 
terrestrial habitat. 
CONCLUSIONS 
Let us now try to see desert animals as part of the general picture 
of evolution. It is sometimes said, or implied, that the terrestrial 
habitat is advantageous in the sense of encouraging biological prog- 
ress. Are we then to think of desert animals, living as they do in 
extreme terrestrial conditions, as being the most progressive of 
animals? 
Such statements are, of course, meaningful only if the term “prog- 
ress” has been defined. The measurement of progress involves a 
criterion of perfection. It is legitimate and to some extent helpful 
to set up such a criterion, provided we realize that the choice is 
arbitrary. Thus we may measure progress, with reasonable objectiv- 
ity, in terms of the range of habitats in which an animal can exist, 
or, and I think preferably, as Herrick (1946) and following him 
Simpson (1950) would have it, in terms of the range and variety of 
adjustments of the organism to its environment—that is, the degree 
of awareness of the environment and of ability to act accordingly. 
If we may accept this measure of progress for the sake of the 
present discussion, the advantages of land life are plain. There is a 
greater variety of habitats on land. Temperature and humidity 
change more rapidly over a greater range and this calls for more 
complex behavior patterns to avoid extremes, or for internal homeo- 
static mechanisms to mitigate their effects. Again, the richer oxygen 
supply and rarer medium permit more rapid and sustained locomo- 
tion, which in turn calls for more acute sense organs—particularly 
for perception at a distance—and more complex behavior patterns to 
cope with the rapid tempo of life. I suspect that the development of 
homeothermy, which is itself a prerequisite for the development of 
intelligence, could never have occurred in aquatic animals because 
