242 EVOLUTION 



and varied : yet the result is scarcely less 

 definite or comprehensive. In the outline 

 of our restatement of the cell -theory as a 

 " theory of the cell cycle " (Chapter III) we 

 have already interpreted such main forms of 

 Protozoa as the rhizopods, the gregarines, the 

 infusors, not from without, as the empirically 

 selected products of spontaneous variations 

 among indefinite possibilities, but from within, 

 as simply the preponderatingly amoeboid, 

 resting, and motile phases of the cell-cycle, 

 three forms determined by the properties of 

 protoplasm itself. 



This conception of life-histories, as physio- 

 logical and not merely structural, rationalizes 

 our animal no less than our vegetable classi- 

 fications. Thus the greatest of all steps in 

 morphological progress, that from the uni- 

 cellular Protozoa to the multicellular Metazoa, 

 is plainly not due to the external selection 

 of the more individuated and highly adapted 

 Protozoan species, but is understood from 

 within, as the union of relatively embryonic 

 and unindividuated cells into an aggregate 

 in which each becomes diminishingly com- 

 petitive as regards its fellows, and increasingly 

 subordinated to the social whole; while within 

 the body thus developed, a series of cells 

 remains relatively undifferentiated as the 



