ANIMAL SURVIVAL IN HOT DESERTS—EDNEY 423 
of the high thermal capacity of water, the scarcity of oxygen, and 
the narrow range and slow rate of temperature changes. 
These then are the possibilities offered by life on land: to be eligi- 
ble for progress along these lines an animal must be terrestrial. But 
this is not to say that it must live in a desert, and we must look 
elsewhere for the reasons why some animals do so. 
There is room, biologically speaking, for amoebae and for men; 
there is room for tapeworms, and shrimps, and land crabs. There 
is doubtless room for hosts of animals which have never been evolved. 
But life continues to exist at all levels of organization and in all 
biological niches once occupied while there is a possibility of existing 
there. There is no compulsion on all organisms to evolve into higher 
organisms. Progress is not inevitable. In other words, organic life 
flows into possible biological niches in all directions; it does not move 
steadily toward one goal. 
The desert is one of these niches. Probably it is one of the most 
difficult of land habitats, but it would be a mistake to draw the con- 
clusion that animals which live there are necessarily more advanced 
than others, in the sense of being able to live in a wider range of 
environments. Camels are clearly not more advanced animals than 
men on our agreed scale of progress. It is broadly true that life has 
evolved from “easier” to “harder” environments, and indeed a desert 
is one of the hardest. The animals which live there are well adapted, 
and their adaptations are interesting to explore. But many of them, 
particularly the arthropods, cannot live in moist cool surroundings. 
If versatility is a measure of progress, then desert animals have sold 
their inheritance for the immediate advantages of specialization. 
As regards adaptation to terrestrial conditions as a whole, mammals 
and arthropods are the two most successful groups, but each group has 
its own secret of success. The mammalian plan is a good one: it 
allows a great variety of habitats to be occupied by a small number 
of relatively generalized species. Mammals can solve water and tem- 
perature problems in a generally applicable way, because they are 
large and can develop homeothermy. Arthropods, on the other hand, 
cannot solve all their water and temperature problems in a general 
way. They are too small, and they must solve some of their problems 
by specializing. Thus one species is specialized to tolerate a high 
range of body temperature, another a low range; one species can 
withstand dry air, another only moist air. Adaptability in arthro- 
pods is a property of the group as a whole rather than of individual 
species. 
And now before I conclude, I must try to answer the inevitable 
question: what is the use of all this work on the physiology of little- 
