On the pampas of Argentina Darwin noted the struct- 

 tural correspondence between such hving forms as the sloths 

 and ant-eaters, and the extinct Megatherium and Glypto- 

 don whose fossil remains he had collected. To him the 

 explanation was blood relationship. 



A study of organic life, therefore, shows relationships, 

 some near, others more remote. It can be compared best 

 to a great tree which branched near the base into two large 

 trunks— the animals and the plants — each dividing again 

 into smaller and smaller branches showing closer and closer 

 relationship at the tips. From this we get our conception 

 of classes, orders, families, and genera. 



The evidence of evolution from comparative structures 

 is strong. The homology of organs in the vertebrates finds 

 no interpretation except on the basis of relationship. The 

 different sets of organs are apparently constructed on the 

 same type. In the course of time the organs have been 

 modified to become better fitted for different kinds of work; 

 for example, the arm of a frog, the paddle of a turtle, the 

 wing of a bird, the fore-leg of a horse, the wing of a bat, the 

 flipper of a whale, and the arm of a man are homologous. 



Sometimes organs that were probably at one time 

 useful and performed functions are now functionless.^.g., 

 vestigial or rudimentary organs, such as the appendix in 

 man, teeth in embryo of the whale-bone whale, ear-mov- 

 ing muscles, etc. 



Darwin likened "vestigial structures" to the un- 

 sounded letters in many words, such as "o" in leopard, 

 "b" in doubt, and "g" in reign, which though functionless 

 tell us something of the history of these words. 



Embryological evidence is also of much weight. The 

 individual during its development (ontogeny) shows many 

 traces of its racial development (phylogeny). Prof. Mar- 

 shall said: "The individual climbs up its own genealogical 

 tree." or, mo'-e technically expressed, "ontogeny tends to 

 recapitulate ;phylogeny" (The Biogenetic Law of Haeckel). 

 The young stages of the higher vertebrates, for example, 

 show remarkable resemblance, and gill-slits occur in the 

 embryos of reptiles, birds, and mammals. 



The recapitulation theory as first enunciated was based 

 on the false view that "in the course of evolution new end 

 stages are added to the ontogeny as it had previously exist- 

 ed." Bu it was shown that "evolutionary changes may 

 affect any portion of the life history, and therefore the 



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