6 THE CHARACTERS OF LIVING BEINGS 



animal or plant. Nutriment supplies the materials for all this 

 increase. But more than material is needed. Neither growth nor 

 any other form of '"vital' activity ever occurs except as a reaction to 

 some appropriate kind of stimulus. This capacity to assimilate 

 nutriment, and to react in certain other peculiar ways to certain 

 definite stimuli, are the marks of a living being. There are, then, 

 three necessary factors, in the absence of any of which growth 

 cannot occur in living beings. The first is tendency (capacity, 

 potentiality) to grow, which determines the kind of development 

 that shall occur. The second is stimulus, which awakens the 

 tendency. The third is nutriment, which supplies the materials 

 for growth, the clay and the straw out of which the bricks are 

 made. Hereditary tendencies are internal factors traceable ulti- 

 mately to the germ-plasm, but both nutriment and stimuli proceed 

 from the world external to the growing structure. Usually not 

 only the kind of growth, but the extent of it, is determined by the 

 hereditary tendencies. Thus, no matter how abundant the supply 

 of nutriment and stimulus, neither mouse nor elephant, for example, 

 can increase beyond a certain size, which, within narrow limits, is 

 definite for the species. In other words, stimuli, which are capable 

 of exciting growth in one period of life (when the animal is young) 

 cease to be capable of exciting it at another (when the animal is 

 adult), no matter how large the supply of nutriment. The limits 

 of growth, however, are much less rigidly drawn in plants, some of 

 which, indeed, seem capable of endless growth, as when propagated 

 by cuttings. It should be noted that nutriment supplies, not only 

 the materials for growth, but acts sometimes, not always as 

 stimulus as well. Thus female bees develop in one way (becoming 

 queens) if one kind of food is supplied, and in another way 

 (becoming workers) if they receive a different kind of food. 1 



1 May I beg the reader to pay especial attention to this paragraph. There is, 

 I believe, nothing in it disputable ; but a great deal that, in effect, is disputed 

 hinges on it. The points to which I wish particularly to call attention are these : 

 (i) An individual can develop only within limits which are predetermined by 

 ' hereditary tendencies ' that have their roots in his germ-plasm. Thus a man 

 could not develop a human limb, nor increase the size of his muscles by use, nor 

 heal a wound of the limb by a scar unless his germ-plasm was of a kind that 

 enabled him to respond to the right stimuli in the right way. (2) An individual 

 makes no growth, develops in no way, unless he is stimulated to develop in that way 

 by the right stimulus, which may be nutriment, or use, or injury, or something 

 else. (3) Nutriment furnishes the materials (the clay and the straw) for all 

 growth. It also, as I say, acts, apparently, as the stimulus for some growth. In 

 other words some growth occurs, apparently, merely because the individual 

 absorbs nutriment : here nutriment acts both as material for growth and as 

 stimulus to it. But in other cases growth occurs only under the stimulus of use 



