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, Hfe 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 total 

 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 

 enviromnent 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 



' L. J. Henderson, The Fitness of the Environment, 1913. 



