CHAPTER II 

 THE EMANCIPATION OP THE PORE-LIMBS 



WE may not here turn aside to inquire into the origin of 

 limbs, nor pause to consider the questions which of neces- 

 sity arise out of the fact that, while all Vertebrates are 

 limited to four limbs, the Invertebrates know no such 

 limitation. 



We will start with the facts as we know them: that 

 Vertebrates possess four outgrowths from the body seg- 

 ments, arranged as a symmetrical pectoral, and a sym- 

 metrical pelvic, pair of limbs ; and that these limbs appear 

 probably in their elemental form as the fins of fishes. 

 We may assume that the most primitive type of Verte- 

 brate limb is an appendage, which is adapted for the 

 purpose of ordered and regulated progression. Limbs 

 may merely propel the aquatic vertebrate body in a 

 definite direction as oars propel a boat; and yet, even 

 when we may regard them as a new acquisition in the 

 Vertebrate phylum, they already subserve other func- 

 tions. Some fishes propel themselves through the water 

 by movements of their fins they "swim"; but the 

 source of real propulsion in a great many is the lateral 

 movements of the tail, the fins serving far more as balanc- 

 ing and regulating organs than as a means of propulsion 

 through the water. This possibility of the limbs develop- 

 ing a balancing function is one that becomes greatly 

 elaborated in the story of the limbs of higher animals. 



We are more immediately concerned here with the 

 limbs of those Vertebrates higher than the fishes; and 

 the type of limb from which we will start our comparisons 

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