THE STUDY OF ZOOLOGY 21 



unit in organization, the individual cells composing it lose their 

 significance as primary units of organization. Sachs recognized 

 this long ago when he said (in 1865) that "cell formation is a 

 phenomenon, very general it is true, in organic life but still 

 only of secondary significance ; at all events it is merely one of the 

 numerous expressions of the formative forces which reside in all 

 matter, in the highest degree, however, in organic substance." 



The view that the organism and not the cell is the unit of 

 organization in living nature is now generally accepted and is 

 known as the organismal theory. On this basis the important 

 difference between protozoan and metazoan animals is not 

 primarily in the number of cells but in the difference in the 

 organization of the protoplasm in the two cases, which in the 

 final analysis means difference in the chemical and physical 

 constitution of the two kinds of protoplasm. 



The problem of cell versus organism in determining the location 

 of organic unity is, of course, a problem only in metazoan animals, 

 for the simple reason that in protozoans the cell and the organ- 

 ism are identical. The criteria for determining unity are 

 primarily physiological. It is a common observation that reac- 

 tions to stimuli are adaptive, i.e., reactions tend to preserve the 

 condition of wholeness in the organism. Isolated groups of cells 

 of metazoan animals as a rule do not survive, or if they do survive, 

 under natural conditions, they tend to organize themselves along 

 the lines of the organism from which they came, rather than as 

 cell units. 



Evolution. — There exists abundant evidence for the belief 

 that the present population of the earth, and the condition of the 

 earth itself, are the results of a process of change or evolution 

 that has extended over enormous periods of time. The only 

 alternative to such an explanation is some form of Special 

 Creation, which raises more problems than it solves. The 

 theory of evolution implies that animals and plants were not 

 always as they are now, but that they have descended from more 

 primitive forms of life through the action of slow processes of 

 divergence which for the most part have been adaptive in 

 character. 



Evolution has been the guiding thought of biology for more 

 than half a century, thanks largely to the painstaking work of the 

 great English naturalist, Charles Darwin, whose publications, 



