CHAPTER XXI 



THE PHENOMENA OF REGRESSION AND OF 

 CONVERGENCE 



Regression by parasitism and fixation Rudimentary organs Pro- 

 gressive and regressive Evolution Functional and non-functional 

 regression Regressive characteristics of the Ammonoids Pheno- 

 mena of total or partial convergence Conclusions. 



ALL naturalists know that under the influence of 

 conditions, among which the parasitic life and fixa- 

 tion are the chief, certain animals belonging to the 

 most varied groups are subject, in the course of 

 their individual life, to those phenomena of arrested 

 development, or even of degradation and degenera- 

 tion, which we call by the general name of phe- 

 nomena of regression. The disappearance of the 

 digestive organs, the loss or complete transforma- 

 tion of the apparatus of locomotion, are the most 

 habitual marks of this regression which may affect 

 other organs, or even almost the whole of the 

 organism. The Cirripedes, such as the Balanae and 

 the Anatifse, are a classical example of these facts ; 

 after having passed through a larval state of the 

 Nauplius type identical with that of the other lower 

 Crustacea, then through a stage similar to the one 

 realized by the adult Cypris, the animal remains 

 stationary, greatly alters its form, and surrounds 



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