THE EPIC OF EVOLUTION 507 



technical details in pigeonholing the thirty thousand or so kinds of 

 flies that occur in the United States alone. It is little wonder that 

 many half brothers turn up in such an extensive fraternity. 



So it comes about that the taxonomist, while still resorting to con- 

 venient pigeonholes in classifying plants and animals, comes more 



"mammals 



acraniotes. ^ Yv^«f;U* birds / 



I c/cbstomes reptiles / / 



I /fishes aTnpbibicoi$p-...™<<<Archaeopte90< 



I I V / I'-y^- Sauropsida. 



\ \ y._^^tf^.....lchthxopsicta 



\ I ^^x^?r. Gnath oStomatcc 



L^^rfr. Craniota 



The theoretical branching of the Craniote tree. X marks the spot where the 



reptile-bird Archaeopleryx roosts. 



and more to picture, in his mind at least, a branching tree as the 

 proper symbol by which to represent the obvious relationships that 

 connecting links indicate. A tree with its trunk giving rise to 

 branches and twigs is, in fact, a perfect diagrammatic picture of the 

 evolutionary process. Such a zoological, phylogenetic tree, includ- 

 ing only vertebrates, however, is shown in the figure, with X marking 

 the spot where Archaeopleryx can comfortably roost. 



In a similar taxonomic tree, enlarged to include all animal creation, 

 a watery floating jellyfish on one of the lower branches might humbly 

 look up to an earthworm, with its wonderfully prophetic head end, 

 while arrogant human beings in the very tree-top look down patron- 

 izingly upon the scatter-brained monkey, and all other biological 

 way-stations. 



Similar taxonomic trees may also be constructed to show what 

 is known about the possible relationship between different groups 

 of plants. 



Finally, in summation it may be repeated that the key to classifica- 

 tion is relationship ; that is, the derivation of one form from another, 

 which is evolution. 



