THE EVOLUTION OF INVERTEBRATES 293 



morphology, and the embryological stages in the develop- 

 ment of the individual. We shall consider first the record of 

 geology, and in order to do this intelligently we shall have 

 to begin far back, for perhaps more than any other science, 

 geology requires of the mind of man vast sweeps of the 

 imagination to form even faint conceptions of the illimitable 

 processes that have brought the earth to its present state. 

 Scarcely less awe-inspiring is the contemplation of the changes 

 that must have taken place in living things, since life began/ 

 in the ocean and on the land. Great as the time and changesl! 

 were before the earth had life upon it, greater still may be^ 

 the time that the processes of evolution have required to I 

 develop all the forms of life in their complexity. 



The Nebular Hypothesis. The earth is one of a number of 

 bodies called planets, which revolve about a central heated 

 body, the sun. The planets, with their attendant moons or 

 satellites, and a number of smaller bodies revolving about the 

 sun in the same direction and nearly in the same plane, con- 

 stitute our solar system. The nebular hypothesis is a theory 

 which involves the suggestion of a common origin for all 

 members of the solar system from a mass of heated gaseous 

 material (Lat. nebula, cloud) in motion, which is supposed to 

 have occupied all the space between the central sun and the 

 orbit of the outermost planet. In the process of cooling and 

 condensation rings of matter were formed, which later broke 

 up into the different planets. 



Archaean Time. The earliest period of the earth's geological 

 history is termed the Archaean Era (Gr. archaios, ancient). 

 At first all the substances, including the water which now 

 covers three quarters of the earth's surface, were held sus- 

 pended in the atmosphere, owing to the high temperature. 

 Later there came a time when the waters condensed, and the 

 surface, cooled still further, permitted the water to cover 

 the rocks of the early crust entirely or in part. Stupendous 



