CHAPTER XXXVI 

 CONTINUOUS AND DISCONTINUOUS EVOLUTION 



THE necessary limitation of the body of living individuals 

 causes the evolution of life on the earth's surface to be 

 represented by a succession of fragmentary masses, instead 

 of being the history of one continuous mass growing inde- 

 finitely. 



One result of this fragmentation is the opening of a wider 

 field to variation. When an individual is once developed 

 its anatomy, in fact, is more or less fixed by its skeleton 

 which limits narrowly possible morphological variations. 

 The reproductive element, being separated from the body, 

 reproduces in all liberty a new individual and new skeleton, 

 while the hereditary patrimony which it carries is able to 

 conform itself better to external conditions. 



A certain number of animals show a similar pheno- 

 menon in their individual existence. Crabs and lobsters, 

 for example, are subject to periodical moulting. These 

 animals are enclosed in a very resistant calcified crust, 

 which opposes itself to their growth as well as to any vari- 

 ations whatever. By the phenomenon of moulting they 

 are relieved of the hard part of their skeleton and, thus 

 reduced to soft substance, take the form of equilibrium 

 which agrees with them. All variation, all modification 

 is limited in such animals to this phenomenon of moulting. 



From a certain point of view, we can compare the specific 



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