434 POPULAR SCIENCE MONTHLY. 



specialization which arises in the higher or many-celled animals, cer- 

 tain alliances of cells or tissues are set apart for respiration alone, and 

 certain others for digestion, while other functions of animal life are 

 relegated to still other cell alliances. Each organ in turn is released 

 from all functions except its own. 



Irritability, or the response to external stimulus, is an attribute 

 of all living organisms. In the method and degree of response varia- 

 tions occur. These variations favorable to the division of labor and 

 the adaptation of the animal to its surroundings are seized and fixed 

 by natural selection. In this way, on the basis of a diffused func- 

 tion, an organ is built up and the organ itself is specialized and per- 

 fected. 



The mind and consciousness of man grow out from the irritabil- 

 ity of the lower animals. They are developed through series of suc- 

 cessive differentiations and integrations. All the higher animals 

 are colonies of co-operating and co-ordinated cells. In such colonies 

 of units the functions of sensation, thought, and motion are rele- 

 gated to series of the most sensitive and most highly organized cells. 

 This alliance of cells is adequate for the work it has to per- 

 form. The brain is always adequate for the mind, for the one is the 

 organ, the other the function, and the development of the two must 

 go on together. 



The intellect of man can not be regarded as the crowning marvel 

 of the " great riddles of life." A marvel is no greater for its bigness. 

 Life is one continuous marvel, without break or end. The human 

 mind is one of life's manifestations. The marvel appears in great 

 or small psychic powers alike, for the great powers of the many- 

 celled brain are produced by the co-operation and specialization of 

 the small powers of the single cell. Nature knows neither great 

 nor small. " God works finer with his hands than man can see with 

 his eyes." The single cell is far from simple. The egg or germ 

 cell carries within itself the whole machinery as well as the whole 

 mystery of heredity. The simplest organism we know is far more 

 complex than the Constitution of the United States. Its adjust- 

 ments, checks, and balances are more perfect. It should in its 

 changing relations be compared rather with the great unwritten con- 

 stitution of civilized society. The laws of society spring from ■the 

 laws governing the development of the single cell. If we knew the 

 latter " all in all," as Tennyson says of the flower, " we should know 

 what God is and man is." 



If we could know all of any life problem to its uttermost detail, 

 we should have the clew to all life. 



Among the protozoa, as already stated, all activities are centered 

 in the single cell which forms the animal unit. Each cell is suf- 



