44 THE THEOKY OF EVOLUTION 



animals and those of the present Siberian or high 

 alpine fauna superposed. 



(4) The phenomena of regression and of convergence 

 (Law of Convergence). 



(a) Regressive Evidence. Many animals experience 

 during their individual lives, under the influence of 

 particular conditions (parasitism for example, or transfer 

 to an established mode of life), a clear depreciation of 

 many organs through regression and reversion. The 

 digestive apparatus can be entirely transformed for 

 instance, in the female of many parasitic crabs (Ler- 

 naiden), 1 as also that of locomotion, which in both cases 

 are no longer purposeful for movement from place to 

 place and therefore disappear (Lernaiden for instance) 

 or serve other purposes (direction of nutriment towards 

 the mouth) as will the species of Lepas, the so-called 

 ' Duck Mussels ' in the Crab group of Centipedise. 



Palaeontology, in the opinion of serious investigators, 

 has now afforded some evidence of the way in which, 

 in the course of geological time, a reversion of some 

 sections of organisms gradually occurred. Thus the 

 presumed parents of the horse of to-day possessed 

 fully-developed lateral toes which now appear only 

 as ' sesamoid bones 9 and are hidden beneath the 

 skin. It is furthermore accepted that the Birds, whose 

 oldest representatives in the Jura and the Chalk forma- 

 tions all possessed teeth, lost them gradually. In 

 young parrots and in the embryos of other birds of 



1 R. Hertwig : LcJirbuch der Zoologie, Jena, 1907, p. 382. 



