THE EVOLUTION OF ANIMAL LIFE 



Organic Evolution 



According to the hypothesis just presented, the original Hving things did not 

 appear as cells, but as much simpler aggregations of molecules possessing, as a 

 consequence of their organization, the most rudimentary properties of life. 

 A great epoch in organic evolution must have been marked by the gradual 

 emergence of organisms in the form of cells, with division of labor between 

 nucleus and cytosome. It may be supposed that this group of primitive cell- 

 like organisms was ancestral to the existing protozoans and unicellular plants, 

 all of which have evolved with great specialization as single-celled organisms. 

 Another line of descent from the primordial unicellular forms attained the 

 multicellular condition, perhaps beginning as persistent colonial aggregations 

 of similar one-celled organisms. In this line the rise of true metazoan charac- 

 teristics involved the differential specialization of individual cells for the 

 performance of particular functions. 



The facts of comparative anatomy and embryology suggest that the ac- 

 quisition of such features as a gut cavity, some kind of body cavity, and 

 bilateral symmetry were subsequent and successive steps in the evolution of 

 the more complex types of metazoan animals. To postulate such broad steps 

 in evolutionary change takes us, in time, far back of the earliest animal 

 fossils; the evidence is suggestive, rather than conclusive. The phylogenetic 

 tree presented in Figure 7.3, page 219, is based largely on such suggestive 

 evidence. Within the more complex phyla, however, which have left a con- 

 siderable fossil record, the evidence for later changes is much more concrete. 

 Several specific examples of such well-documented sequences will be presented 

 later in this chapter. 



Evidence for the Fact of Evolution 



We may now consider the evidence that has led biologists to conclude that 

 the innumerable species of animals and plants now living, and all others that 

 have lived in past times, have arisen through an evolutionary process. This 

 evidence is subject to the same limitations as all historic data, but it has 

 proved convincing because so many separate lines of evidence permit the same 

 conclusions. Perhaps no single one of the principal lines would be considered 

 sufficient in itself, but taken together, and reinforcing each other, they are 

 very convincing. 



EVIDENCE FROM DISTRIBUTION 



Geologic distribution refers to the distribution in time of ancient forms of 

 life, as revealed by paleontological Studies of their fossil remains (Fig. 20.1). 

 It may be contrasted with geographic distribution, which is the spatial or 



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