1 88 Changing Plant and Animal Nature 



species throughout the ages, with the advance of man 

 through the forest and the prairie, with the change in climate, 

 with the rise of continents, with the subsidence of land, 

 with the advance and retreat of glaciers, with the meander 

 of a stream, and so on. Just as a relatively small change in 

 the conditions of life may offer an opportunity for the rapid 

 increase of one species, it may make for the rapid extermina- 

 tion of another. There is a virtual increase then in the fit- 

 ness of the former, and of the unfitness of the latter, with- 

 out any essential change in their characters and processes. 



Evolution and Progress ' 



The term evolution has been used loosely to suggest an 

 advance or improvement. This, however, is not a necessary 

 implication of the facts of transformation of species, for 

 sometimes successive stages have shown deterioration. It is 

 specialization that progresses in time. The orchids appear 

 later than the lilies. The social insects appear later than the 

 cockroaches. The advance in specialization has indicated in 

 general a more and more precise adaptation to restricted 

 conditions of living, with corresponding advantages of 

 economy. The mammals, for example, can maintain them- 

 selves by producing a very small number of offspring, 

 whereas some of the fishes and insects produce hundreds of 

 thousands of eggs a season without increasing in the number 

 of individuals from year to year. The flowering plants that 

 are pollinated by insects maintain themselves by producing a 

 very small quantity of pollen compared to the masses pro- 

 duced by the wind-pollinated trees. 



On the other hand, extreme specialization has its dis- 

 advantages. The plants and animals of the past that have 

 died out are frequently the most highly specialized among 

 their contemporaries. In our own time, we see many species 

 of orchids dying out because they depend for their repro- 

 duction upon the visits of particular insects, which are not 

 present in sufficient numbers to insure the formation of seed 



