ECOLOGY 2S7 



those whoso reactions are useful, or neutral, or at most only slight I3' 

 detrimental. 



The Environment as a Whole. — The relations of animals to each other 

 and to the physical environment have been considered separately, but it 

 should be pointed out that it is the sum total of the physical and biological 

 conditions affecting the life of an animal which make up the environment 

 as a whole. The specific complex preferred by a species constitutes its 

 habitat, and a change in any detail may determine the existence, abun- 

 dance, and distribution of the form. The physical conditions, the plants, 

 and the associated animals comprise a complex of environmental condi- 

 tions so closely intern^latcd that variations in any feature may aisturb 

 the balance and render difficult or impossible the existenco of a particular 

 form. In this connection it should be stated that the physical conditions 

 everywhere are changing and that the environments are thus unstable. 

 Modern geography teaches that streams change with age in respect to 

 the nature of the bottom, swiftness, etc., that the lakes tend to become 

 converted into dry land, and that land may be elevated and subsequently 

 lowered by erosion with consequent changes in soil, climatic conditions, etc. 

 The geographical changes bring about changes in animal life; and since 

 they are orderly a rather definite succession of associations can be shown 

 to accompany them. For example, under normal conditions the asso- 

 ciations in lakes come to be supplanted by those of ponds, the pond 

 associations give place to those of marshes, and the marsh associations 

 yield to land associations. The animal communities change in this order 

 because this is the geographical sequence which leads to the disappearance 

 of freshwater lakes. 



Methods and Aims of Ecology. — The work of the ecologist may be 

 defined as the determination of the animal associations and the relations 

 of the members of each association to the environment, for the purpose of 

 discovering the principles in accordance with which the associations are 

 formed. In the gathering of data two groups of students have become at 

 least partially differentiated: first, those who place chief emphasis upon 

 the determination of associations and, second, those who pursue studies 

 of a physiological nature, experimenting upon animals to determine the 

 effects of varying intensities of the physical conditions which are found 

 in the natural habitat. The differentiation among zoologists is not as 

 pronounced as it is in the field of plant ecology. There seems to be a 

 growing understanding among zoologists that while a knowledge of the 

 associations is necessary, their composition is an end result of the reactions 

 of the inhabitants to the features of the environment, and that the subject 

 to yield results must concern itself with these reactions. 



Ecology can hardly be considered a science comparable to morphology' 

 and embryology. It has a number of points of contact with other 

 sciences, but it is apparently essentially intermediate between physiology 



