TRANSACTIONS OF THE SECTIONS. 185 



in the meau temperature either of the eai-th generally or of the two hemispheres ; but 

 they may, wlieu combined -with changes of the direction of the eai'th's axis caused 

 by the precession of tlie equinoxes and nutation, lead to exaggeration of the 

 extremes of heat and cold, or to their diminution ; and this would appeal- to sup- 

 ply the means of explaining the observed facts, though doubtless the detailed ap- 

 plication of the conception will long continue to give rise to discussions. Rii-. 

 (Jroll, in his book entitled ' Climate and Time,' has recently brought together with 

 much research all that can now be said on this subject; and the general correctness 

 of that part of his conclusions which refers to the periodical occm-rence of epochs 

 of gi-eatly increased winter cold and summer heat in one hemisphere, combined 

 with a more equable climate in the other, appears to me to be fully established. 



These are the considerations which are held to prove that the inorganic 

 structure of the globe through all its successive stages — the earth beneath our 

 feet, with its varied surface of land and sea, mountain and plain, and with its at- 

 mosphere which distributes heat and moistm-e over that surface, — has been evolved 

 as the necessary result of the original aggi-egation of matter at some extremely re- 

 mote period, and of the subsequent modification of that matter in condition and 

 form under the exclusive operation of invariable physical forces. 



From these investigations we carry on the inquiry to the li'sdng creatures found 

 upon the earth ; what are their relations one to another, and what to the inorganic 

 world with which they are associated ? 



This inquiry first du'ected to the present time, and thence carried backwards as 

 far as possible into the past, proves that there is one general sj'stem 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 geolo- 

 gical research. The phenomena of life, as thus ascertained, are included in the 

 organization of living creatm-es, and their distribution in time and place. The 

 common bond that subsists between all vegetables and animals is testified by the 

 identity of the ultimate elements of which they are composed. These elements are 

 carbon, oxygen, hydrogen, and nitrogen, with a few others in comparatively small 

 quantities; the v^hole of the materials of all living things being found among 

 those that compose 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 difficulty that arises in determining the nature of the 

 distinctions between them. Among the more highly developed members of the two 

 great branches of living creatures, the well-known similarities of structure observed 

 in the various groups indicate a connexion 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 creatm-es in neighbouring 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 accessibilit}' diminishes, or the conditions of climate differ, the like- 

 ness of the forms within them becomes continually less apparent. The plants and 

 animals existing at any time in any locality tend constantly to diffiise themselves 

 around that local centre, this tendency being controlled by the conditions of 

 climate, &c. of the surrounding area, so that imder certain unfavourable conditions 

 diffusion ceases. 



The possibilities of life are further seen to be everywhere directly influenced by 

 all external conditions, such as those of climate, including temperatm-e, humidit}', 

 and wind ; of the length of the seasons and days and nights ; of the character of 

 the surface whether it be laud or water, and whether it be covered by vegetation 

 or otherwise ; of the natm-e of the soil ; of tlie presence of other living creatures, 

 and many more. The abundance of forms of life in different ai-eas (as distinguished 

 from number of individuals) is also found to vary greatly, and to be related to the 

 accessibility of such areas to immigration from without ; to the existence, within 

 or near the areas, of localities offering considerable variations of the condidc ns that 

 1875. 14 



