526 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1917. 



these animals can only find the optimum conditions when the slow 

 process of humus formation reaches a certain degree of cumulati re 

 development. Under such circumstances this later stage must be pre- 

 ceded by antecedent processes, and restoration of the balance is long 

 delayed. Some adjustments take rjlace so quickly that little can be 

 learned of the stages through which they pass. There are, however, 

 many slow processes which afford an abundance of time for study; 

 in fact some are too slow to study during a lifetime. The processes 

 which are moderately slow are often particularly illuminating be- 

 cause all stages are frequently so well preserved that comparison is a 

 very useful method of study ; the slowness of a process has a certain 

 resolving power, as it were, recalling the influence of a prism upon 

 a beam of white light, which reveals many characteristics obscure to 

 direct vision. A study of the processes of adjustment among ani- 

 mals is a study of an important phase of the problem of maintenance. 

 The continued process of response will, if circumstances permit, lead 

 to a condition of relative adjustment, or to a balancing among all 

 the factors in operation. 



7. ECOLOGICAL UNITS FOR STUDY. 



In the study of animal responses many different units are avail- 

 able, and a brief consideration of these will aid in an understanding 

 of the methods which are useful. Because the animal body has been 

 found to be composed of a single cell or a multitude of cells, a com- 

 mon belief has grown up that the cell is the natural unit for study. 

 This opinion seems to be due to overlooking the fact that there is just 

 as much reason for considering the whole animal as the unit. The 

 unicellular animals are whole animals as truly as they are cells, and 

 in multicellular animals the activity of single cells means little inde- 

 pendently of the animal as a whole. It thus seems that ecologically 

 at least the smallest valuable unit for study is the individual animal. 

 The responses of the individual, as a kind of animal, to its condition 

 of existence form the basis for what may be called individual ecology. 

 Animals which are related by descent from common ancestors, as a 

 community of social animals (e. g., an ant colony), or taxonomic 

 units, such as genera, families, orders, etc. (e. g., fish, birds, catfishes, 

 and salamanders), are also units which may be studied ecologically. 

 Some of these hereditary units are, ecologically, fairly homogeneous, 

 as, for instance, when a taxonomic unit is equally distinct ecologi- 

 cally: e. g., the woodpeckers with their arboreal habits. In other 

 cases the taxonomic unit contains animals of great ecological diver- 

 sity, as in the case of beetles, which possess almost unlimited ecolog- 

 ical diversity, including littoral, aquatic, subterranean, and arboreal 



