EVOLUTION 231 



point at which our modern flowering plants fii'st appeared upon 

 the earth. No other evidence for evolution is quite so convincing 

 as are these tangible remains of extinct organisms. 



Taxonomic Evide7ice. — The general character and classification 

 of the plant and animal kingdoms also bears testimony that 

 their present state is the result of descent, with progressive modifi- 

 cation, from earlier types. A study of the external and internal 

 structure of living things makes it clear that they are not hap- 

 hazard and random in their characteristics, but that they fall 

 into well-marked groups of similar forms, the members of which 

 show definite resemblances to each other. All similar indivi- 

 duals we class together as a species. A number of species resem- 

 ble one another so much, and are so different from anything else, 

 that we place them together as a genus. A number of genera, 

 in the same way, stand apart as a family. Families are united 

 into orders, orders into classes, and so on. We can understand 

 this grouping of similar species and their union into progressively 

 larger aggregations if we regard the organic world as a huge 

 "genealogical tree", its members related to one another — some 

 nearly, some remotely — by ties of descent, the ''twigs" repre- 

 senting species, which unite into larger and larger branches as we 

 trace them back to the main trunk. These facts, which make the 

 science of taxonomy possible, are unexplainable otherwise. 



Morphological Evidence. — Equally significant are certain facts 

 which morphology presents. Many organs exist today in a 

 state evidently useless to the plant or animal possessing them 

 and for which it is hard to account unless we look upon them as 

 vestiges or remnants of structures which once had a use but have 

 lost it during the course of evolution. Vestigial stamens, petals, 

 sepals, stipules, and leaf blades, as well as various functionless 

 internal structures, are of frequent occurrence in plants, and there 

 are many similar instances in the animal kingdom. Their 

 presence can be explained only if we assume that they once were 

 well developed and functional but that evolutionary^ progress, 

 which makes them necessary no longer, has resulted in their 

 gradual degeneration. 



Evidence from Geographical Distrihuiion. — Impressive evidence 

 in favor of evolution is presented by the facts of geographical 

 distribution. Most plant species are not widely dispersed over 

 the earth's surface, or even over that part of it in which condi- 

 tions are well suited for their growth. The golden-rods, for 



