EXPLANATION OF THE FIGURES. 



FRONTISPIECE. The diagram opposite the title page is intended to present, 

 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, of 

 different shades, each of which is subdivided by circles, indicating the num- 

 ber of formations of which they are composed. The whole disc is divided 

 by radiating lines into four segments, to include the four great departments 

 of the Animal Kingdom ; the Vertebrates, with Man at their head, are placed 

 in the upper compartment, the Articulates at the left, the Mollusks at the right, 

 and the Radiates below, as being the lowest in rank. Each of these com- 

 partments 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 to re- 

 present the primitive egg, with its germinative vesicle and germinative dot 

 (278), indicative of the universal origin of all animals, and the epoch of life 

 when all are apparently alike (275, 276). Surrounding this, at the point 

 from which each department radiates, are placed the symbols of the several 

 departments, as explained on page 124. The zones are traversed by rays 

 which represent the principal types of animals, and their origin and termination 

 indicates 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 different geo- 

 logical ages. Thus, in the class of Crustaceans, the Trilobites appear to 

 commence in the earliest strata, and to disappear with the carboniferous 

 formation. The Ammonites also appeared in the Silurian formation, and did 

 not become extinct before the deposition of the Cretaceous rocks. The 

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

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



