Living Organisms 737 



developmental changes through which the ancestors of our modern horse 

 are believed to have gone are shown in the evolution of the horse (Figs. 

 359 and 360). 



2. Evidences From Taxonomy (Classification). — A comparative study 

 of the various species of animals and plants reveals a very great simi- 

 larity; in fact, so great that it is difficult to decide where one species with 



Fig. 362. — The Archaeopteryx, A, a reptilelike bird of the Upper Jurassic 

 period compared with the pigeon (Columha livia), B. (From Lull: Organic 

 Evolution. By permission of The Macmillan Company, publishers.) 



its variations ends and another species with its variations begins. The 

 intergrades (divergent individuals with a certain species) frequently are 

 very similar functionally and structurally to those of a closely related 

 species. When we attempt to classify similar types of organisms, we see 

 clearly the close anatomic and physiologic relationships between many 

 of them. What is the explanation for these similarities? Are living 



