!86 



NA TURE 



[September 23, 1909 



it enabled them to leave the sea, because they carried with 

 them the watery environment which was essential for the 

 normal activity of their constituent cell units. The assump- 

 tion of a terrestrial existence on most parts of the earth's 

 surface involved, however, the exposure to greater ranges 

 of temperature than was the case in the sea, and indicated 

 the necessity for still further increase in the range of 

 adaptation. Every vital process has its optimum teinpera- 

 lure at which it is carried out rapidly and effectively. At 

 or a little above freezing point the chemical processes con- 

 cerned in life are suspended, so that over a wide range 

 of the animal kingdom there must be an almost complete 

 suspension of vital processes during the winter months, 

 and at all times of the year a great dependence of the 

 activity of these processes on the surrounding temperature. 

 It is evident that a great advantage in the struggle for 

 existence was gained by the first animals which succeeded 

 in securing thermal as well as chemical constancy of 

 environment for their cells, thus rendering them indepen- 

 dent of changes in the external medium. It is interesting 

 to note that the maintenance of the temperature of warm- 

 blooded animals at a constant height is a function of the 

 higher parts of the central nervous system. An animal 

 with spinal cord alone reacts to changes of external 

 temperature exactly like a cold-blooded animal, the activity 

 of its chemical changes rising and falling with the tempera- 

 ture. In the intact mammal, by accurately balancing heat 

 loss froi./ the surface against heat production in the 

 muscles, the central nervous system ensures that the body 

 fluid which is supplied to all the active cells has a tempera- 

 ture which is independent of that of the surrounding 

 medium. These are fundamental examples of adaptation 

 effected by creation of an environment peculiar to the 

 animal. Numberless others could be cited which differ 

 only in degree from the activity of man himself. In some 

 parts of this country, for instance, the activity of the 

 beaver in creating an artificial environment has until lately 

 been more marked than that of man himself. We are 

 not justified, then, in regarding mankind as immune to 

 the operation of natural forces which have determined the 

 sequence of life on the surface of the globe. The same 

 laws which have determined his evolution and his present 

 position as the dominant type on the earth's surface will 

 determine also his future destiny. 



We are not, however, dealing with or interested in 

 simple survival. Lower forms of life are probably as 

 abundant on the surface of the globe as they were at any 

 time in its history. Survival, as Darwin pointed out, is 

 a question of differentiation. When in savage warfare a 

 whole tribe is taken captive by the victorious enemy, the 

 leaders and fighting men will be destroyed, while the 

 slaves will continue to exist as the property of the victors. 

 Survival, then, may be determined either by rise or by 

 degradation of type. Success involves the idea of domin- 

 ance, which can be secured only by that type which is the 

 better endowed with the mechanisms of adaptation re- 

 quired in the struggle against other organisms. 



.'\mong the many forms of living matter which may 

 have come into being in the earlier stages of the history 

 of the earth, one form apparently became predominant and 

 iTiust be regarded as the ancestor of all forms of life, 

 whether animal or vegetable, viz., the nucleated cell. The 

 almost complete identity of the phenomena involved in 

 cell division throughout the living kingdom indicates that 

 all unicellular organisms and all organisms composed of 

 cells have descended from a common ancestor, and that 

 the mode of its reproduction has been impressed upon 

 all its descendants throughout the millions of years which 

 have elapsed since the type was first evolved. The 

 universal distribution of living cells renders it practically 

 impossible for us to test the possibility of a spontaneous 

 abiogenesis or new formation of living from non-livinc 

 matter at the present time. We cannot imagine that all 

 the various phenomena which we associate with life were 

 attributes of the primitive life stuff. Even if we had such 

 stuff at our disposal, it would be difficult to decide whether 

 we should ascribe the possession of life to it, and there 

 is no doubt that any such half-way material would, directly 

 it was formed, be utilised as pabulum by the higher types 

 of organism already abounding on the surface of the 

 globe. 



NO. 2082, VOL. Si] 



Integration and Differentiation. 

 An important step in the evolution of higher forms was 

 taken when, by the aggregation of unicellular organisms, 

 the lowest nietazoon was formed. In its. most primitiv.e 

 forms the metazoon consists simply of a cell colony, .but 

 one in which all individuals are not of equal significance. 

 Those to the outer side of the mass, being exposed • to 

 different environmental advantages from those within, must 

 even during the lifetime of the individual have acquired 

 different characteristics. Moreover, the sole aim of such 

 aggregation being to admit of cooperation by differentia- 

 tion of function between the various cell units, the latter 

 become modified accoicling to their position, some, cells 

 becoming chiefly alimentary, others motor, and others -re- 

 productive. Cooperation and differentiation are, however, 

 of no use without coordination. Each part of the organism 

 must be in a position to be affected by changes going on 

 in distant parts, otherwise cooperation could not be 

 effected. This cooperation in the lowest metazoon seenis 

 to be carried out by utilisation of the sensibility to chemical 

 stimuli alreadv possessed by the unicellular organism. We 

 have thus coordination by means of chemical substances 

 (" hormones ") produced in certain cells and carried thence 

 by the tissue fluids to other cells of the body, a mechanism 

 of communication which we find even in the highest 

 animals, including man himself. To such chemical stimuli 

 we mav probably ascribe the accumulation of wandering 

 mesoderm cells — i.e. phagocytes — in an organism such as 

 a sponge, around a seat of injury or any foreign substance 

 that has been introduced. By this mechanism it is possible 

 for distant parts of the body to react to stimulation of 

 any one part of the surface. Communication by this 

 means is, however, slow, and may be compared to the 

 state of affairs in civilised countries before the invention 

 of the telegraph, when messengers had to ride to different 

 parts of the kingdom in order to arouse the whole nation 

 for defence or attack. 



Foresight and Control. 

 Increased speed of reaction ■ and therefore increased 

 powers in the struggle for existence were obtained when 

 a nervous system was formed, by a modification of the 

 cells forming the outer surface of the organism. By the 

 growth of long processes from these cells a conducting 

 network was provided, running through all parts of the 

 body and affording a channel for the rapid propagation of 

 excitation from the surface to the deeper parts, as well 

 as from one part of the surface to another. From this 

 same layer were produced the cells which, as muscle fibres, 

 would act as the motive mechanism of the organism. 

 Thus, from the beginning, the chief means of attack or 

 escape were laid down in close connection with the surface 

 from which the stimuli were received. A further step in 

 the evolution of the nervous system consisted in the with- 

 drawal of certain of the sensory or receptor cells from trie 

 surface, so that a specially irritable organ, the central 

 nervous system, was evolved, which could serve as a dis- 

 tributing centre for the messages or calls to action initiated 

 by changes occurring at the surface of the body, .^t its 

 first appearance this central nervous system would hardly 

 deserve the .'pithet of " central," since it formed a layer 

 Iving some distance below the surface, and extending over 

 a considerable area ; though we find that very soon there 

 is an aggregation of the special cells to form ganglia, 

 each of- which might be regarded as presiding over the 

 reactions of that part of the animal in which it is situated. 

 Thus in the segmental worm-like animals a pair of ganglia 

 is present in each body segment, and the chain of ganglia 

 are united by longitudinal strands of nerve fibres to form 

 the ganglionated cord, or central nervous system. 



Such a diffused nervous system, in which- all ganglia 

 were of equal value, could, however, only act for the 

 common weal of the whole body when a reaction initiated 

 by stimulation at one part was not counteracted- by an 

 opposing reaction exi ited from another part of the surface. 

 For survival it Is necessary that in the presence of danger, 

 i.e. an environment threatening the life of the individual 

 or race, the whole activities of the organism should be 

 concentrated on the one common purpose, whether of 

 escape or defence. This could be effected only by making 

 one part of the central nervous system predominant over 

 all other parts, and the part which was chosen for this 



