4 BOTANY 



substance becomes more isolated, and, consequently, independence 

 of action in plants, as compared with animals, is diminished. By 

 means of the green colouring matter, plants have the power of 

 producing their own nutritive substances from certain constituents 

 of the air and water, and from the salts contained in the soil, and 

 are thus able to exist independently ; while animals are dependent, 

 directly or indirectly, for their nourishment, and so for their very 

 existence, on plants. Almost all the other differences which dis- 

 tinguish plants from animals may be traced to the structure of 

 plants, or to the manner in which they obtain their food. Another 

 characteristic of plants is the unlimited duration of their ontogenetic 

 development, which is continuous, at the growing points, during 

 their whole life. That none of these criteria are alone sufficient 

 for distinguishing plants from animals is evident from the fact that 

 all the Fungi are devoid of green pigment, and, like animals, are 

 dependent on substances produced by green plants for their 

 nourishment. On the borderland of the two kingdoms, where all 

 other distinctions are wanting, phylogenetic resemblances, accord- 

 ing as they may indicate a probable relationship with plants or 

 animals, serve as a guide in determining the position of an 

 organism. 



While it is thus impossible to distinguish sharply the two great 

 groups of living organisms from one another, a distinction between 

 them and lifeless bodies is readily recognised. Living organisms are 

 endowed with the quality of IRRITABILITY, in which all lifeless 

 bodies are deficient. External or internal stimuli influence living 

 organisms to an activity, which is manifested in accordance with 

 the requirements and conditions of their internal structure. Even 

 in the smallest known organisms the manifestations of life are 

 occasioned by a similar sensitiveness to external or internal stimuli. 

 It is, therefore, probable that the simplest living beings must have 

 possessed essentially simpler properties than any organisms now 

 known, which would enable us to connect them with non-living 

 substances. The substance which serves as a basis for all develop- 

 ment must be supposed to have had an inorganic origin. So far as i- 

 actually known, however, all living organisms have arisen only from 

 similar organisms. So far as experience has shown, spontaneous 

 generation is unknown. In the olden times it was a common 

 supposition, which ARISTOTLE himself held, that even highly organised 

 animals and plants could originate from sand and mud. In the 

 same degree that knowledge of the actual development of living 

 organisms was extended, the previously accepted cases of spontaneous 

 generation became more and more restricted, and were finally 

 limited to intestinal worms which could not otherwise, it was 

 thought, be accounted for, and to microscopic organisms, the origin 

 of which also was not understood. Now, for such organisms the 



