220 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



a single organ, or correlatively to a group of organs 

 or apparatus. 



A certain number of facts of regression are easily 

 explained by the necessities of functional adaptation. 

 I shall confine myself to recalling the well-known 

 examples of the reduction of the lateral digits in 

 the Equidse and the Ruminants, with the object of 

 increasing the speed ; and that of the pelvis of the 

 Sirenians, the Cetacea, and the Pythonomorphs, 

 with the object of concentrating the effort of nata- 

 tion on the anterior limb and the tail. The Marine 

 Turtles offer, as regards this, a rather special com- 

 plication. The Chelonidse of pelagic habitat have 

 lessened by regression the weight of the bony case 

 which encloses them, by the aid of empty spaces or 

 fontanelles placed on the sides of the carapace and 

 in the centre of the breastplate ; but the Athecse, 

 or Spargidae, under the influence of a return to land 

 life, have retained the reduced shield of the 

 Chelones by replacing it by a secondary shield 

 composed of polygonal dermic plates superposed, 

 without being welded together, on the rudiment of 

 the primary one. 



In many other cases regression occurs without 

 apparent object, and as a normal consequence of 

 the evolution of a group. In the Cephalopod class 

 in particular, a whole series of facts have been 

 interpreted as regressive characteristics. The ex- 

 ample most often quoted is that of the Ammonites 

 of the upper Chalk (Tissotia), which were at first 

 connected with the Ceratitae on account of their 

 very simple line of suture, composed of whole 

 saddles and slightly denticulated lobes. According 



