PLANTS, AND INANIMATE MATTEE. 87 



ganized. Later naturalists found that these definitions were 

 unsatisfactory. Then importance was attached to the 

 absorption of nutriment by fibres at the lower ends of 

 plants and the presence in animals of a mouth above or 

 anteriorly, leading to a stomach. Next, naturalists sought 

 a reliable distinction between plants and animals in the 

 exhalation of carbonic acid by animals and oxygen by plants. 

 With increasing knowledge of new forms of life, all these 

 distinctions were found to be unsatisfactory, and new ones 

 were suggested, depending on, e.g., the nature of the cell, 

 the properties of protoplasm, the presence or absence of 

 chlorophyll, and the nature of the food of animals and 

 plants. To-day, however, the difficulties are confined 

 chiefly to the numerous very small forms of life of which 

 Aristotle and even Linnaeus and many later naturalists had 

 no knowledge. With respect to such small forms of life. 

 Sir Kay Lankester says : " When, however, we come to the 

 very lowest unicellular microscopic forms of life, there is 

 greater difficulty in assigning some of the minuter organisms 

 to one side or the other, and to some extent our decision in 

 the matter must depend on the theory we may provisionally 

 adopt as to the nature of the earliest living material, which 

 was the common ancestral matrix from which both the 

 Plant series and the Animal series have developed." * 



It is clear, therefore, that Aristotle, when he attempted 

 to determine a boundary line between animals and plants, 

 became the pioneer of a work which has engaged the atten- 

 tion of numerous investigators right up to the present time. 

 He was not aware of the complicated nature of the pheno- 

 mena which it would be necessary to understand before so 

 difficult a task could be completed, but he made a creditable 

 attempt. That he knew only comparatively few forms of 

 life, and that he had great difficulty in deciding on the nature 

 of some, the position of which has long been determined, do 

 not deprive him of the credit of being the first to indicate 

 how a boundary line may be drawn between plants and 

 animals. 



A Treatise on Zoology, part i. 1909, p. xiv. 



