218 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



itself with a true calcareous shell of many pieces, 

 which for a long time caused this order of the lower 

 Crustacea to be ranked among the Molluscs. The 

 fixation of the free larva of certain Lamellibranchs, 

 such as the oysters and the Pectines, likewise brings 

 about regressive modifications of structure, although 

 less important. Jackson has shown that the young 

 forms of these Molluscs were provided with byssus 

 and with two adductor muscles, while the adult 

 forms lose by regression the byssus and the anterior 

 muscle, the disappearance of which is accompanied 

 by other notable changes in the general form of the 

 body. 



These very various modes of adaptation to different 

 environments have led in the living animals of the 

 present day to innumerable cases of reduction 

 which betray themselves by the presence of rudi- 

 mentary organs, whose signification can only be 

 established by referring to the genealogical history 

 of the group. Such are the bony stylets hidden 

 under the skin and representing the rudiments of 

 the lateral toes which were unconfined in the 

 ancestors of the Horse ; such again, the traces of 

 dental germs which we discover in the beaks of 

 young Parrots and are an atrophied remnant of the 

 teeth persisting into the adult age in the Birds of 

 Secondary times ; such is the pelvis reduced and 

 deprived of limbs, of the Sirenians and Cetacea, or 

 the pineal eye of the Hatteria, hidden under an 

 opaque dermic plate and recalling the functional 

 Median eye of several palaeozoic Reptiles. Darwin 

 and many other naturalists have, with reason, 

 insisted on these well-known facts as one of the 



