EXPLANATION OF THE FIGURES. 



FRONTISPIECE. The diagram opposite the title-page is intended to pre- 

 sent, at one view, the distribution of the principal types of animals, and 

 the order of their successive appearance in the layers of the earth's crust. 

 The four Ages of Nature, mentioned at page 190, are represented by four 

 zones, each of which is subdivided by circles of different shades, indicat- 

 ing the number of formations of which it is composed. The whole disc 

 is divided by radiating lines into four segments, to include the four 

 great departments of the animal kingdom ; the Vertebrata are placed in 

 the upper compartment, the Articulata at the left, the Mollusca at the 

 right, and the Radiata below, as being the lowest in rank. Each of these 

 compartments is again subdivided to include the different classes belonging 

 to it, which are named at the outer circle. At the centre is placed a figure 

 representing the primitive egg, with its germinative vesicle and germinative 

 dot C 436), indicative of the universal origin of all animals, and the epoch 

 of life when all are apparently alike. Surrounding this, at the point from 

 which each department radiates, are placed the symbols of the several de- 

 partments, as explained on page 337. The zones are traversed by rays 

 which represent the principal types of animals ; their origin and termi- 

 nation indicate the age at which they first appeared or disappeared ; all 

 those which reach the circumference being still in existence. The width 

 of the ray indicates the greater or less prevalence of the type at dif- 

 ferent geological ages. Thus, in the class of Crustaceans, the Trilobites 

 commence in the earliest strata, and disappear with the carboniferous 

 formation. The Ammonites also appeared in the Silurian formation, 

 and became extinct with the deposition of the Cretaceous rocks. The 

 Belemnites appear in the lower Oolitic beds ; many new forms commence 

 in the Tertiary; a great number of types make their appearance only in 

 the Modern age ; while only a few have continued from the Silurian, 

 through every period to the present. Thus, the Crinoids were very nu- 

 merous in the Primary Age, and are but slightly developed in the Tertiary 

 and Modern Age. It is seen, at a glance, that the animal kingdom is 

 much more diversified in the latter, than in the earlier ages. 



Below the circle is a section, intended to show more distinctly the re- 

 lative position of the ten principal formations of stratified rocks ( 648), 

 composing the four great geological ages ; the numerals corresponding to 

 those on the ray leading to Man, in the circular figure. See also figure 376. 



