60 



OUTLINES OF EVOLUTIONARY BIOLOGY 



higher animals distributed amongst thousands of millions of 

 such units, arranged, so to speak, in regiments and armies, each 

 group with its own duties to perform and all co-operating for the 

 common good under the supervision and control of the central 

 nervous system. The marvellous perfection of the whole 

 machinery is the result of that differentiation and division of 

 labour which was first rendered possible by the union and co- 

 operation of the individuals 

 of a protozoon family to form 

 a multicellular body. 



Turning now to the higher 

 plants, we shall find that, in 

 accordance with their much 

 lower degree of functional 

 activity, their organization 

 is far less elaborate than in 

 the higher animals. In cor- 

 relation with their stationary 

 habit all those organs and 

 tissues which are specially 

 concerned with locomotion 

 are absent, and in further 

 correlation with this character 

 there are no nervous system 

 and no special organs of sense. 

 Of the functions concerned 

 with the life of the individual 

 that is, other than repro- 

 ductive functions that of 

 nutrition is alone highly 



FIG. 24. Diagram of a Neuron, 

 (From Hertwig.) 



Nervenfortsatz, a nerve-fibre coming off 

 from the body of the cell. 



developed. The entire plant 

 is little more than a piece of 

 apparatus for extracting carbon, water and mineral salts from the 

 air and soil, and converting these, with the aid of the sun's rays, 

 into organic substances. Although the higher plants often attain 

 a much larger size than any animals, this does not indicate a 

 higher degree of organization, for it is brought about simply by 

 the repetition of similar parts such as roots, branches and 

 leaves and the accumulation of dead cell-walls in the form of 

 wood and bark. As we have already seen, a green plant, instead 

 of spending the energy which it derives from the sun on its own 



