200 THE POPULAR SCIENCE MONTHLY. 



of greatly-increased winter cold and summer beat in one hemisphere, 

 combined with a more equable climate in the other, aj^pears to me to 

 be fully established. 



These are the considerations which are held to prove that the in- 

 organic structure of the globe through all its successive stages the 

 earth beneath our feet, with its varied surface of land and sea, moun- 

 tain and plain, and with its atmosphere which distributes heat and 

 moisture over that surface has been evolved as the necessary re- 

 sult of the original aggregation of matter at some extremely remote 

 period, and of the subsequent modification of that matter in condi- 

 tion and form under the exclusive operation of invariable physical 

 forces. 



From these investigations we carry on the inquiry to the living 

 creatures -found upon the earth : what are their relations one to an- 

 other, and what to the inorganic world with which they are associ- 

 ated ? 



This inquiry, first directed to the present time, and thence carried 

 backward as far as possible into the past, proves that there is one gen- 

 eral system of life, vegetable and animal, which is coextensive with 

 the earth as it now is, and as it has been in all the successive stages 

 of which we obtain a knowledge by geological research. The phe- 

 nomena of life, as thus ascertained, are included in the organization 

 of living creatures, and their distribution in time and place. The 

 common bond that subsists between all vegetables and animals is tes- 

 tified by the identity of the ultimate elements of which they are com- 

 posed. These elements are carbon, oxygen, hydrogen, and nitrogen, 

 with a few others in comparatively small quantities ; the whole of 

 the matex'ials of all living things being found among those that com- 

 pose the inorganic portion of the earth. 



The close relation existing between the least specialized animals 

 and plants, and between these and organic matter not having life, 

 and even with inorganic matter, is indicated by the difliculty that 

 arises in determining the nature of the distinctions between them. 

 Among the more highly-developed members of the two great branch- 

 es of living creatures, the well-knowu similarities of structure ob- 

 served in the various groups indicate a connection between proximate 

 forms which was long seen to be akin to that derived through descent 

 from a common ancestor by ordinary generation. 



The facts of distribution show that certain forms are associated 

 in certain areas, and that as we pass from one such area to another 

 the forms of life change also. The general assemblages of living 

 creatures in neighboring countries easily accessible to one another, 

 and having similar climates, resemble one another ; and much in the 

 same way, as the distance between areas increases, or their mutual 

 accessibility diminishes, or the conditions of climate differ, the like- 

 ness of the forms within them becomes continually less apparent. 



