CHAPTER XX 



THE LAW OF SPECIALIZATION OF PHYLETIC 

 BRANCHES 



Progressive specialization of Branches Specialization of organs with 

 different functions The runner's foot of the Ungulates The 

 swimming limbs of the Sirenians The natatory paddle of the 

 Ichthyosaurs Production of offensive or defensive weapons 

 Non-functional specializations : line of suture and uncoiling of the 

 Ammonites The Rudistte The Senility of Branches. 



AT the same time that the mutations of a same 

 branch increase in size during their evolution, they 

 are influenced by another law, which is that of a 

 more or less marked specialization in one direction. 

 This phenomenon did not escape the sagacity of 

 Cope, who formulated it in a rather different form 

 and so to speak, inversely : i.e. as the law of non- 

 specialization. Organic types which are not special- 

 ized are alone, according to him, capable of an 

 ulterior evolution. Since the time of Cope, the 

 progressive specialization of phyletic branches has 

 been the object of very many researches among 

 all groups of fossil animals ; but here again, as in 

 the question of increase in size, the most convincing 

 proofs will be derived from the vertebrate animals. 

 Generally speaking, the specialization does not 

 apply to the organism as a whole, but only to one 



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