drugs, which are mainly plant products, 

 there are the artificially synthesized chemo- 

 therapeutic agents, such as salvarsan (which 

 was used for many years against Treponema 

 pallidum, the syphilis parasite), the sulfon- 

 amides (which are effective against many 

 bacterial infections), and several synthetic 

 antimalarial compounds. 1 Also to be men- 

 tioned are the antibiotic compounds, like 

 penicillin, streptomycin, and aureomycin, 

 which are extracted from certain molds, bac- 

 teria, and other fungi. These advances are 

 very encouraging, but the problem of find- 

 ing a particular compound that is effective 

 against a specific parasite is not an easy one. 

 Most compounds that are toxic to a parasite 

 are also toxic to the cells and tissues of the 

 host. If the compound is administered in 

 amounts that are adequate to eliminate the 

 parasite, the host itself is unable to tolerate 

 the dose. In each case, therefore, the essential 

 problem is to find a compound that is at the 

 same time specifically toxic to the cells of the 

 particular parasite and relatively nontoxic 

 to the cells of the host (man). 



Disease Vectors. Various blood-sucking 

 creatures frequently serve as transmitters of 

 disease, as has been acknowledged since 1893, 

 when Theobald Smith first proved that the 

 blood-dwelling parasite of Texas cattle fever 

 is transmitted from animal to animal by the 

 bite of a common tick. Subsequently, as a 

 result of many investigations, the importance 

 of vector organisms, especially insects, in rela- 

 tion to human disease became increasingly 

 apparent. Today, in fact, it is common knowl- 

 edge that various mosquitoes are mainly re- 

 sponsible for the spread of malaria, yellow 

 fever, dengue, and filaria infections; that the 

 tsetse fly transmits African sleeping sickness; 

 that lice carry typhus; that fleas spread the 

 "black plague"; and finally that ticks are the 

 main purveyors of Rocky Mountain spotted 

 fever. Moreover, a study of vectors has greatly 

 aided man's fight against disease. Mosquito 



1 Antimalarial research during World War II led 

 to the discovery of several compounds that have 

 proved to be more effective than atabrine. 



Ecology and Evolution - 581 



control made it possible to complete the 

 Panama Canal, after a first attempt had been 

 abandoned owing to the ravages of yellow 

 fever; and more recently, a drive against lice 

 enabled the American Army to overcome a 

 grave epidemic of typhus in the Naples area, 

 during the Italian campaign of World War 

 II. Likewise, the development of modern in- 

 secticides, such as DDT, has provided a new 

 and powerful weapon against the insect- 

 borne diseases. 



The Abiotic Environment. Throughout the 

 universe living matter has been found only 

 on the earth, a relatively small planet in the 

 solar system. In fact, no other region has 

 been discovered where the environment is 

 chemically and physically suitable for the 

 existence of living things, although probably 

 other life-supporting planets do exist through 

 the universe (p. 187). Protoplasm is an 

 extremely sensitive and unstable form of 

 matter. It can exist and perpetuate itself 

 only under a very limited set of physical and 

 chemical conditions such as have prevailed 

 on the earth for many ages. 



The general fitness of the terrestrial envi- 

 ronment in relation to living organisms is 

 determined by a wide variety of conditions. 

 However, only four physical factors, namely 

 temperature, light, gravity, and pressure, 

 and only one chemical factor — the water sup- 

 ply — will be considered in this brief account. 

 Temperature. A tremendous variation of 

 temperature occurs within the universe — 

 from the frigid stillness of interstellar space, 

 where the temperature approaches absolute 

 zero — to the shattering heat of the sun, where 

 the temperature exceeds 10,000,000 degrees 

 centigrade. The range of temperature that is 

 tolerable to living things is very limited, 

 however. Few organisms can survive except 

 at temperatures between 0° and 50° C; and 

 if such temperatures had not prevailed for 

 many years over large areas of the earth, life 

 as it is today could not have evolved. 



The sensitivity of living things to cold and 

 heat derives mainly from the fact that water 

 and proteins are main components of living 



