3. All living things, whether repre sented by the smallest unit or by such a complex organism as man, are indi viduals and in some way differ from the other individuals of the species. Such individuality may be evidenced either in obvious differences of structure or by differences in reaction. This holds true as far as methods enable us to study individuality, but such studies cannot be made on all the members of a group of the smallest bacteria. This individuality is due to variation in char acter or arrangement which may be either molecular or atomic, of the living material, for it is impossible to think of precisely the same structure not giving the same reaction in a common environ ment. On the other hand, every organ in a mass of such an inorganic sub stance as sodium chloride is of precisely the same character and gives the same reactions.
4. Living matter possesses to a re markable degree the power of adapta bility to environmental conditions. Dif ferent living things show great differ ences in this power of adaptation, in some it is extremely limited, certain parasites for instance find a suitable en vironment only within certain cells of a narrow animal species, while in others life is possible in a wide range of envi ronment. As a rule the more complex the organism the greater is the capacity of adaptation.
5. Living matter undergoes constant change, there is constant chemical de composition, breaking down of high molecular forms into lower, constant oxidation and formation of CO:. Such constant destruction is compensated by equally constant formation of new ma terial. The organism rows when the formative processes more than balance the destructive, and wastes when the reverse takes place. Growth is compli cated, for the added material when taken into the cell differs from its substance and must be formed into cell material and organization by the physical and chemical action within the cell. Such growth by intussusception was formerly supposed to distinguish cell growth from growth of crystals, but recent investiga tions have shown that the distinction is not absolute. The building material is usually, in part at least, derived from some form of life, save in the case of plants containing chlorophyll which are able by means of light energy to build up complex substances from the simpler, and this power is shared by certain of the bacteria. Certain organisms are able to pass into what is known a3 latent life, in which all activity ceases, but can be resumed when the environ ment becomes favorable. This state is associated with great reduction of the water content and the resumption of ac tivity is inaugurated by rapid absorp tion of water.
6. Relation of temperature to life. All chemical processes in living matter and in most cases life itself are possible only within certain temperature limits. In a condition of latency, temperatures which are destructive to active life can be withstood. The temperature limits within which active life is possible are somewhat above the freezing point of water and somewhat below the point at which albumen coagulates. In the
highly organized mammals the temper ature limits are narrow and not more than 10° C. The temperature limits for latent life are extremely variable, rang ing from 3 to 4 degrees above absolute zero to considerably above the boiling point of water, as shown by the spores of certain bacteria.
7. All living matter undergoes cyclic change. All forms of life come from pre-existing forms. The living thing grows until it reaches the form and size characteristic of the species, remains in this stage for a variable and often very short period, then the processes of de struction overbalance the processes of growth and there is final disruption of structure, or death. The continuation of life is provided for by the separation of parts of the living matter in which again growth overbalances destruction. In some cases this renewal of life is a simple process, in others extremely com plicated. From this it is evident that the living thing is to be regarded as a most complicated and intricate chemical mechanism whose activity is due to forces acting upon it from without. There is no force inherent in living mat ter, there are no vital forces independent of and differing from the cosmic forces; the energy manifested by living matter, whatever its form, is counterbalanced by the energy which it receives. It is perfectly true that we cannot explain certain of the forms of energy which the living matter gives off by compari son with such mechanisms as can be produced, but no other mechanism can be compared in complexity of structure and range of activity with the living thing. Attempts to bridge the difficul ties of comparison of living activity with mechanisms have been made by the use of such terms as vital force, and there is no objection to the use of such terms provided it be understood that by them we merely express ignorance. Nothing is known of consciousness save as the manifestation of a form of vital ctivity; nor does it seem to be open to investigation by the methods of science. It is not known whether it is a general attribute of living matter. Nor is any thing known of the primary origin of life. At sonic period of the evolution of the earth, hydrogen, oxygen, and car bon may have come together in such combinations as water and carbonic acid as would give combinations with other substances to form protoplasm. Tho primary difficulty is to explain the origin of protoplasm; given this, evolution will account for all the forms of life. So far as we know living matter is im mortal; death is due to the action of conditions which so change its character that the peculiar activities which char acterize life are no longer possible. It is not inconceivable that ultra-micro scopic forms of life may be originating at the present time.