66 



PRINCIPLES OF ANIMAL BIOLOGY 



two theories of the relation of parts to wholes fare very unequally. Only 

 a few modern representatives of the one type of change may be selected, 

 while very many are available to represent the other. 



The Organismal Theory. — The organismal theory is that which treats 

 living things primarily as wholes, to which the parts are subordinate. 

 In accord with this theory, the evolution of complex organisms from 

 simple ones should start with an increase in complexity in some animal or 

 plant while it is still a single cell. Much differentiation in the structures 



of the cytosome must have occurred. It would 

 be expected also that the nucleus would have 

 divided into many nuclei without corresponding 

 divisions of the cytosome. That is, a multi- 

 nucleate cell would have arisen out of a uninu- 

 cleate one. Protoplasm containing many nuclei 

 without separating cell membranes is known in 

 a number of animals and plants and is called a 

 syncytium. Voluntary muscle cells (page 95) in 

 the higher animals have many nuclei, and the 

 developing eggs of insects (Fig. 172) pass through 

 a stage in which there are many nuclei before 

 cell membranes begin to appear. One cannot, 

 however, think of these very complex metazoan 

 structures as remnants of an evolutionary stage 

 which most of the other metazoa have passed. 

 To have any possible significance as representa- 

 tive of a step in evolution, the syncytium should 

 be some rather simple organism. Vaucheria 

 (Fig. 46), one of the simple plants, is syncytial, 

 and there are several other plants. Good ex- 

 amples are lacking among animals. The organismal theory is thus 

 not well supported by living representatives of the stages for which 

 it calls, though this lack can hardly be regarded as a fatal objection to 

 the theory. 



The Colonial Theory. — If one regards organisms as the sum of their 

 component parts, the natural supposition is that multicellular animals 

 and plants arose through some form of colony formation. Cells multi- 

 plied by division and then, instead of falling apart as they do among the 

 protozoa, they clung together in groups. Such colonies could be formed 

 before any of the cells became any more greatly differentiated than the 

 single cell had been. The differentiation and increase in complexity 

 could then follow in a succession of steps. The multicellular condition 

 comes first, the complexity later, rev(n-sing the order expected from 

 the organismal concept. This way of deriving the metazoa has the 



Fig. 46. — Vaucheria, a 

 simple plant illustrating 

 a syncytium or multinu- 

 cleate cell. {From Sharp, 

 "Introduction to Cytology.") 



