250 



GENERAL BIOLOGY 



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Fig. 155. Diagram illustrating 

 the skeleton of the foreleg of 

 the modern horse, (g) and a 

 series of fossil forms (a to /) ap- 

 parently illustrating its phylo- 

 geny; A, humerus; r, radius; ul, 

 ulna; c, carpus; ph, phalanges; 

 a, Orohippus; b, Mesohippus; 

 C, Miohippus; d, Protohippus; 

 e, Pliohippus; /, Equus, the 

 modern horse. 



one of the most 

 famous of these, 

 illustrating the 

 phylogeny of the 

 horse. Possible 

 stages in the de- 

 velopment of the 

 single toed hoof of the horse as found in fos- 

 sils are represented in figure 155. And it may 

 confidently be believed that more such series 

 would be found were the fossils better 

 known. Palaeontology is continually fill- 

 ing the gaps between the sundered groups 

 of recent species. 



The persistence of the unspecialized. — In 

 spite of the abounding testimony of palaeon- 

 tology that throughout the history of organ- 

 isms the strong and the well-equipped have 

 frequently dropped out of the race for life, 

 and in spite of the obvious structural advant- 

 ages of the higher types as we have studied 

 them in the first part of this chapter we still 

 have amoebas and other very simple organ- 

 isms flourishing in our midst. How have 

 they withstood the stress of competition 

 with forms that appear to be so much better 

 equipped ? Why have they not all been ex- 

 terminated? It may not be possible to say 

 how any particular organism has succeeded, 

 but we do well to remember, first, that size 

 and specialization and organization have 

 their disadvantages as well as their advant- 

 ages, and second, that most specializations 

 occur in adaptation to removal into new fields 



a 



