LECTUKE XII. 

 GEEAT STEPS IN ORGANIC EVOLUTION. 



§ 1. The Origin of Organisms upon the Earth. § 2. The Nature of 

 the First Organisms. § 3. Establishment of Diverse Types of 

 Cellular Organisation. § 4. The Divergence of Green Plants. 

 § 5. The Making of Bodies. § 6. The Divergence of the 

 Sexes. § 7. Progressive Differentiations and Integrations. 

 § 8. Rise and Progress of Backboned Animals. § 9. The As- 

 cent of Man. § 10. General Impressions of Animate Evolu- 

 tion. 



The largest and most overwhelming idea in all science 

 is that the system of Nature in all its complexity, intricacy, 

 multitudinousness, and harmony has come to be as it is 

 from apparently simple beginnings — from something like a 

 nebula if we go back to inorganic genesis, from a crowd of 

 invisible microbes if we begin with the primordial organisms 

 on the cooling earth. We say the word evolution so often 

 that we are apt to get dull to the overpowering grandeur 

 and undeniable mysteriousness of the process. It may not 

 be altogether unprofitable to attempt the impossible, — a short 

 review of the great achievements. 



§ 1. The Origin of Organisms upon the Earth. 



As every one knows, there was a time when the tempera- 

 ture of our earth was beyond the endurance even of tho 

 mythical salamander. It was far too high to admit of 

 the existence of forms of life like those we know, or can 

 even imagine, and we need not speculate about others. There 



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