8 INTRODUCTION. II. 



Nutrition and assimilation are associated in all living actions, being coeval with 

 the birth of a living being, and ceasing only upon its death. 



During life, particles of the living structure become effete, and are removed by 

 consumption or exuration, through the agency of the oxygen of the atmosphere. 

 This process has been confounded with that of respiration, a function of especial 

 organs, the lungs, branchiae, and tracheae, which exist in higher animals only ; it 

 is really secondary to the more important process of life, exuration. Exuration 

 occurs in plants as well as in animals; in germination of the seed and in inflorescence 

 it is very evident. In the growing plant, exuration is usually masked by the pecu- 

 liar character and activity of the process of nutrition ; but, at night, when the 

 nutrition of the plant is at rest, the exuration becomes marked in the evolution of 

 carbonic acid. 



Development and growth are definite in each species of living being. 



Reproduction perpetuates the structure as well as the species of the living being. 



Death commences with life in the destruction of effete molecules of structure; it 

 is the cessation of all life-phenomena in the individual, or is the last phenomenon 

 of life. 



To live, requires certain indispensable conditions never absent from life ; always 

 preceding it. These consist of the specific components of the living body together 

 with the constant presence of water, air, and a definite range of temperature. 



The constituent matter of living beings necessarily precedes the phenomena of 

 life. 



Without water there could be no movement to indicate life. 



Air is necessary to exuration. No living being is found out of its influence. 

 The minutest radicle of a plant never penetrates into the earth beyond the access 

 of air. 



The range of temperature necessary to life is between 35 F. 1 and 135 F. 2 



Life cannot exist independently of any one of the above-mentioned conditions. 

 In very many instances, the removal of certain of the indispensable conditions of 

 life-action may take place without the destruction of the power of living when 

 these conditions are restored. Thus, many plants and animals, seeds and eggs, 

 may be dried ; yet, upon supplying moisture to them, with all the other conditions, 

 they will again present the characteristic phenomena of life. 



The indispensable conditions of life are susceptible of a great variety of modifica- 

 tion, within a definite range, without its destruction. 



Accompanying a variation of the essential conditions of life, is presented the 



1 The so-called red-snow, Protococcus nivalis, Agardh., an algous plant of polar and alpine regions, 

 grows and reproduces only upon thawing snow, though it may be found beneath virgin snow and in a tem- 

 perature far below zero; nevertheless, in such circumstances it has ceased all activity, and may remain 

 so for a long period. The plant is remarkably indestructible. I have a specimen contained in melted 

 snow-water, yet alive and of a red color, December, 1852, which was brought by the enterprising traveller, 

 Dr. E. K. Kane, U.S.N., from Cape Beverly, latitude 76.10, during the Griunell expedition for 1850-51, 

 in search of Sir John Franklin. 



8 Certain algae grow in thermal springs, of the temperature of 117 F. 



