84 I The Process of Evolution 



vectors. Everyone knows of the instances of pollinating insects 

 carrying the pollen grains (male gamete-producing plants) from 

 flower to flower on their legs or bodies. Less familiar are those 

 orchids in which the flower resembles the abdomen of a female 

 fly or wasp so closely that males of the mimicked species attempt to 

 copulate with the flower. The pollen is carried, in tiny bags or 

 pollinia, from one flower to another on the end of the abdomen of 

 the male insects. 



Similar situations are not unknown among animals. An interesting 

 instance is that of the adult human botfly (Dermatobia hominis), 

 which catches mosquitoes and attaches eggs to their bodies before 

 releasing them. The eggs hatch when the mosquito lands on the 

 warm skin of a man, and the larvae burrow in and start to develop. 

 This fly parasitizes a number of mammals other than man. 



Besides the interactions among plants and animals commonly ob- 

 served in the temperate zone, there are less well-known examples 

 of extreme intricacy in the tropical rain forest. On the branches of 

 the giant trees, plants of various kinds accumulate water and soil 

 among their leaves. In this specialized niche the larvae of mosquitoes 

 and of frogs hatch, grow, and metamorphose. The mosquito fauna 

 is stratified in part because of the distribution of the epiphytic plants 

 in which they grow. In the same forests lives the three-toed sloth, 

 the hairs of which are colored greenish by symbiotic algae. The 

 sloth moth Bradypodicola hohneli spends its entire life on the sloth, 

 its larvae presumably feeding upon the algae. Explaining the evo- 

 lutionary history of associations such as these, and others perhaps 

 even more bizarre, is a challenge to the evolutionist interested in 

 the structure of the ecosystem. 



Changes in climatic patterns strongly aflFect the kinds of organisms 

 that can exist in an area. Arctic fossils of tropical plants and amphibia 

 testify to warmer times in the past, and long-empty desert cities to 

 changes in rainfall pattern or soil fertility. Years of commercial 

 grazing, with no return of essential elements to the soil from decay- 

 ing plants and animals, have changed the nutritional characteristics 

 of many areas of the Great Plains of the United States, with a re- 

 sultant change in the flora and fauna. English-sparrow populations 

 in our cities have become much smaller since the disappearance of 

 the horse and its seed-laden droppings. Grasslands have diminished 

 in some areas as roads and other sorts of fire control increase. 



Man's activities in transplanting organisms have provided many 

 striking instances of change in the influence of "other organisms." 

 The imported cabbage butterfly, Pieris rapae, has increased in North 

 America at the expense of our native cabbage butterfly, P. protodice, 

 presumably by out-eating it. Storksbill, mustard, and wild oats 



