BRIEF REVIEW OF THE SYSTEM OF LIFE. 413 



The map is from the geological chart of C. H. Hitchcock, with changes 

 from later publications. The blank area on the eastern border comprises 

 Archaean, Cambrian, and Lower Silurian rocks, not yet having their limits 

 defined. 



The progress of the life of the globe is one of the two great subjects that 

 come before the student, in the following part of this Manual, treating of 

 Historical Geology. By way of introduction to it, a short chapter on its 

 system of structures is here introduced. 



BRIEF REVIEW OE THE SYSTEM OF LIFE. 



GENERAL CONSIDERATIONS. 



1. Life. — Some of the distinctions between a living organism and inorganic or min- 

 eral substances have been mentioned. Recapitulated, with additions, they are : — 



(1) The living being has, as the fundamental element of its structures, visible cells, 

 containing fluids or plastic material. 



(2) It enlarges by means of imbibed nutriment, through a process of development ; 

 and not by mere accretion or crystallization. 



(3) It has the faculty of converting nutriment into the various chemical compounds 

 essential to its constitution, and of continuing this process of assimilation as long as the 

 functions of life continue ; and it loses this chemical power when life ceases. 



(4) It passes through successive stages in structure, and in chemistry, from the simple 

 germ to a more or less complex adult stage, and finally evolves other germs for the contin- 

 uance of the species ; instead of being equally perfect and equally simple in all its stages, 

 and essentially germless. 



There is, therefore, in the living organism, something besides mere physical forces, or 

 the chemistry of dead nature — something that ceases to be when life ceases. There is a 

 vital condition, in which molecules have powers that lead to resulting seed-bearing struc- 

 tures, widely different from those of inorganic nature, and standing on altogether a higher 

 level. There is a power of development, an architectonic power, that not only exalts 

 chemical results, but evolves a diversity of parts and structures, and a heritage of ancestral 

 qualities, of which the laws of material nature give no explanation. 



2. Vegetable and Animal life. — The vegetable and animal kingdoms are the mutu- 

 ally dependent sides or parts of one system of life. The following are some of their dis- 

 tinctive characteristics : — 



(1) Plants take nutriment into the tissues by absorption ; animals have a mouth, and 

 receive food into a sac or stomach. Exceptions to this feature occur in animal life only 

 in the lowest microscopic forms and certain parasitic kinds ; and the most of these extem- 

 porize a mouth and stomach whenever any particle of food comes in contact with the outer 

 surface, so that even here the food is digested in an interior cavity. Certain insectivo- 

 rous plants "digest" animal material, but the process is not necessary to growth, with 

 small exceptions. 



(2) Plants find nutriment in carbonic acid, appropriate the carbon, and set free oxygen, 

 a gas essential to animal life ; animals use oxygen in respiration, and set free carbonic 

 acid, a gas essential to vegetable life. (The amount of carbonic acid given out by plants 

 in respiration is too small to need consideration here.) 



(3) Plants take inorganic material as food, and turn it into organic ; animals take 

 this organic material thus prepared (plants), or other organic materials made from it (ani- 

 mals), finding no nutriment in inorganic matter. 



