378 THE LAW OF EVOLUTION CONTINUED. 



shelving beaches, over which the tide ebbed and flowed to 

 great distances. 



Respecting the characteristics classed as geological, we 

 may draw parallel inferences. While the Earth's crust was 

 thin, -mountain-chains were impossibilities : there could not 

 have been long and well-defined axes of elevation, with 

 distinct water-sheds and areas of drainage. Moreover, the 

 denudation of small islands by small rivers, and by tidal 

 streams both feeble and narrow, would produce no clearly- 

 marked sedimentary strata. Confused and varying masses 

 of detritus, such as we now find at the mouths of brooks, 

 must have been the prevailing formations. And these could 

 give place to distinct strata, only as there arose continents 

 and oceans, with their great rivers, long coast-lines, and 

 wide-spreading marine currents. 



How there must simultaneously have resulted more 

 definite meteorological characters, need not be pointed out 

 in detail. That differences of climates and seasons grew 

 relatively decided as the heat of the Sun became distin- 

 guishable from the proper heat of the Earth; and that the 

 production of more specific conditions in each locality was 

 aided by increasing permanence in the distribution of lands 

 and seas; are conclusions sufficiently obvious. 



132. Let us turn now to the evidence furnished by 

 organic bodies. In place of deductive illustrations like the 

 foregoing, we shall here find numerous illustrations which 

 have been inductively established, and are therefore less 

 open to criticism. The process of mammalian development, 

 for example, will supply us with numerous proofs ready- 

 described by embryologists. 



The first change which the ovum of a mammal under- 

 goes after continued segmentation has reduced its yelk to a 

 mulberry-like mass, is the appearance of a greater definite- 

 ness in the peripheral cells of this mass; each of which ac- 

 quires a distinct enveloping membrane, These peripheral 



