THE BACKGROUND OF DARWINISM—ADAPTATIONS 189 
admirably discussed by Professor Lawrence J. Henderson in a stimu- 
lating volume.: 
Henderson points out that the environment, no less than organ- 
isms, has had an evolution. The particular environmental complex 
as it exists today is absolutely unique. There is hardly an element of 
the effective environment that could be changed without causing the 
extinction of life or at least a transformation of it so profound that it 
might not be life at all as we know life. Water, for example, has a 
‘dozen unique properties that condition life. Carbon dioxide could not 
be replaced by any other substance. The properties of the ocean are 
so beautifully adjusted to life that we marvel at the exactness of its 
fitness. Finally, the chemical properties of carbon, hydrogen, and 
oxygen, the most abundant elements, are equally unique and unre- 
placeable. In brief, given the environment as it is, life could not be 
other than it is. The evolution of the environment and the evolution 
of organisms have gone hand in hand, or perhaps we might better say 
hand in glove, for this better expresses the idea of mutual fitness. 
Within the realm of the general environment as conceived ‘by 
Henderson there are almost innumerable special environments due to 
particular combinations of the various environmental units. Within 
the aquatic environment; for example, there are variations such as 
differences in salinity, varying from extreme saltiness to almost tota] 
lack of salt; there are inshore conditions and open-sea conditions; there 
are surface conditions and those at relatively great depths; and 
there are great differences due to temperature. Similarly on land, 
there are surface conditions, subterranean conditions, caves, deserts, 
forests, plains, mountains, arctic, tropical conditions, and many others, 
No two areas on land are precisely similar in all respects. All of this 
makes for a corresponding multiplicity of animal and plant forms. In 
the case of plants the action of the environment is remarkably direct; 
for the plant cannot get away from a fixed environment. If the 
environment undergoes material change, the plant’s only response is a 
structural one. For example, if plants that are accustomed to a rela- 
tively humid climate are grown in the desert they develop numerous 
xerophytic adaptations such as small leaves with greatly diminished 
transpiration surface, a thick epidermis, hairs, or spines, small stature, 
deep-root system, and other similar protections against the inimical 
desert conditions. Similarly, plants accustomed to grow in relatively 
tL. J. Henderson, The Fitness of the Environment, 1913. 
