177 



ment. Only when all the factors which influence the develop- 

 ment of an organism combine to act in the most favourable 

 way can an ideally healthy development become possible. 

 In nature, however, this is probably never realized, except 

 perhaps for very limited periods in the life of any particular 

 organism, and, with the gradually increasing predominance 

 of injurious over beneficial factors, we get at first a condi- 

 tion which we recognise as disease and eventually death. 

 Seeing, however, that an ideally healthy development, for even 

 limited periods in the life-history of any particular organism, 

 is rarely, if ever, possible, it is obviously very difficult 

 to define exactly what we mean by disease. We have seen 

 that the ordinary course of the normal development of a 

 plant in itself necessitates the death of considerable portions 

 of the plant-body, or even of the entire plant in cases where 

 death naturally follows the production of flowers and seed, 

 yet here we can hardly speak of disease. For practical pur- 

 poses, however, we may regard as diseased any condition of 

 the plant, or of any part of it, which, unless ameliorated, will 

 lead to the obviously premature death of the plant, or of some 

 part of it. 



155. In Part IV above, it has Strugg'e for 



been noted that what we call the struggle for existence Existence - 

 is responsible for the premature death of organisms, and 

 that ,every organism, in order to maintain itself, has to 

 struggle with other living organisms and to fight against, 

 such factors as unfavourable climate, excessive heat and 

 cold, drought and so on, see p. 148. When it is considered 

 that these factors affect various organisms in very different 

 ways, we begin to realize how complex the relations 

 are which bind together the organic world. Hence when 

 studying the diseases of a particular plant, w3 must 

 not only consider the effect exercised upon it directly by such 

 factors as soil, moisture, temperature, light and such like, 

 but we must also discover how other organisms, plants as 

 well as animals, affect it, what factors influence the develop- 

 ment of all such correlated organisms, and in what way. To 

 illustrate how two organisms, between which at first sight we 

 should say there was absolutely no connection, may be de- 

 pendent on each other for their existence, we cannot do 

 better than take the following well-known example given by 

 Darwin : 



" Humble-bees alone visit red clover, as other bees cannot 

 reach the nectar. * * Hence we may infer as highly 



