ECOLOGICAL BACKGROUND AND GROWTH BEFORE 1900 



27 



(populations) are selected as units. As with 

 many other phases of biology, Darwin's 

 work gave direction to population studies 

 without containing much that was strictly 

 concerned with this particular field. 



Farr, as we have seen, returned in .1875 

 to his discussion of problems related to the 

 human population of England as revealed 

 by the accumulated vital statistics. He saw 

 clearly that a decrease in death rates and 

 a resulting increase in longevity do not 

 necessarily lead to an increase in popula- 

 tion, since, as he cogently remarks, the 

 associated birth rate may fall to an equiva- 

 lent extent. He knew that in man, as in 

 other organisms, the possibility of popula- 

 tion increase in geometrical ratio exists; but 

 (and here Malthus had erred) so also may 

 the means of man's subsistence. Not only 

 had the population of the United States of 

 America doubled itself every tAventy-five 

 years for a century and a half, but the 

 means of human subsistence had also in- 

 creased in geometric ratio and at an even 

 greater rate. This must frequently hold true, 

 since the plants or animals on which man 

 feeds can increase (or decrease) even more 

 rapidly than longer-lived, slow-breeding 

 man. Restated in terms of the pyramid of 

 numbers, which Farr did not do, this can 

 be turned into another general principle. 



A close consideration of the ideas of 

 Malthus concerning population growth and 

 control, and of Darwin concerning evolu- 

 tion, would seem to require oscillations in 

 the populations of what would now be 

 called key-industry animals and in those of 

 the carnivores that feed upon them. Spen- 

 cer (1863) wrote about this "rhythm in 

 number of each tribe of animals and plants" 

 in approximately modem terms. We have 

 recently been reminded by Elton (1942) 

 that knowledge of mouse plagues, which 

 represent an outstanding oscillation in 

 nature, dates back to early Hebrew history 

 and that such plagues were well known to 

 Aristotle, Theophrastus, Pliny, and others of 

 the classical period. They were obser\'ed 

 somewhat critically during the last decades 

 of the nineteenth century, the formative 

 years for much of modern ecology. 



Knowledge concerning populations had 

 another line of ancestry in the biometri- 

 cians, Galton, Weldon, and Karl Pearson. 

 Aside from Weldon's work (1898) on the 

 relation of the survival of crabs in Plymouth 

 Harbour (England) to the width of the 



carapace, and a few similar papers, these 

 men contributed disappointingly little 

 directly to the knowledge or theory of 

 populations. It remained for an American 

 disciple, Raymond Pearl, to make the transi- 

 tion from biometry to population studies 

 that somewhat approximates the ecological 

 approach to the subject. Like his redis- 

 covery (with Reed, 1920) of Verhulst's 

 logistic curve and his eflFective use of that 

 curve as a quantitative expression of poten- 

 tial rate of increase and of environmental 

 resistance, these developments by Pearl 

 came too late to aflFect the early rise of 

 ecology. Their modern aspects and their re- 

 lations to other phases of present day ecol- 

 ogy will be treated later (p. 46). 



ECONOMIC BIOLOGY 



Many population studies have a strong 

 economic tiend, and the pressure of eco- 

 nomic problems not only accelerated the de- 

 velopment of an adequate basis for modern 

 ecology, but continues to stimulate eco- 

 logical development today. Three broad 

 economic interests of man— fisheries, agricul- 

 ture, and certain aspects of medicine— are 

 closely related to ecology. The need for 

 more precise information concerning food 

 fishes and the conditions of their existence 

 has been one of the potent drives in the 

 study of the ecolog)' of aquatic habitats. 

 The relation between ecology and agricul- 

 ture is even more obvious; many of the 

 environmental relations of plants were stud- 

 ied in the eighteenth and nineteenth 

 centuries, as well as in earlier and more 

 recent times, because of their direct bearing 

 on agricultural problems. The data re- 

 viewed by Abbe (1905) were discovered 

 primarily because of their immediate 

 economic application, and Abbe's compre- 

 hensive review was itself similarly moti- 

 vated. 



On the animal side, an important element 

 in the background of ecology came from 

 work with insects in relation to man-growTi 

 crops and to the control of diseases of do- 

 mestic animals and of man. Precise 

 summaries of the history of these develop- 

 ments will be found in books devoted to 

 economic and to medical entomology, 

 especially those on the history of entomol- 

 ogv, notably Howard (1930) and Essig 

 (1931). The treatment here wdll be sug- 

 gestive rather than comprehensive. 



The regulation of population size of 



