A DEFINITION OF EVOLUTION 



among the major groups ot organisms, but it becomes very much more 

 frequent when the study is extended to the lower levels of classification. 

 As soon as the need for parallel rungs at many levels became apparent, 

 with parallel series above them, it immediately was evident that a tree 

 would make a much better diagram, and this was generally accepted long 

 before Darwin. 



Now everyone understands that the parts of a real tree are related to 

 one another in a branching fashion because the whole organism is the 

 product of growth from a single seed, growth accompanied by branching 

 and differentiation. Independent creation and secondarv union of the 

 many parts would be unthinka1:)le. The taxonomic tree is not strictly com- 

 parable to an actual tree, for here the processes of branching and dif- 

 ferentiation are not generally amenable to direct observation. But the 

 analogy cannot be avoided: the fact that no other type of diagram can 

 symbolize the data of taxonomy so readily as a tree strongly suggests that, 

 like a real tree, the tree of life owes its branching character to organic 

 growth and differentiation— in other words, to evolution. Pre-Darwinian 

 biologists were unable to understand why it was that classification seemed 

 to fall into a tree-like pattern, yet they agreed that it was so. Long aware- 

 ness of this fact undoubtedly helped to prepare the way for the ultimate 

 acceptance of Darwinism. 



A final characteristic of the tree of life deserves special consideration. 

 In any group, there are likely to be some members which are simpler and 



r < 



Figure 6. Ten Specimens of Warblers Representini; Five Species of Dendroica. 

 Note the gradual dccpniing of color from left to right. Mcmhcrs of the same species 

 are not always adjacent in this series. The transition appears e\en more gradually in 

 color. ( Specimens loaned by the Zoological Museum of the University of Notre Dame. ) 



50 



