THE president's ADDRESS. 171 



or by the decay of other liv^ing organisms. Further, among 

 both animals and plants we meet with the phenomena of 

 parasitism, which need not be discussed at present ; it is sufficient 

 to point out that a parasite is an organism which invades or 

 attacks some other organism in order to rob it of its proteids. 



When the Protista are considered from the point of view of 

 their metabolism, it is found that in one order, the Flagellata, 

 all the four modes of metabolism just mentioned occur amongst 

 organisms obviously most closely allied. Some have chlorophyll 

 and live like green plants, others have no chlorophyll and live 

 as saprophytes, others again like animals, or as parasites. But 

 an even more remarkable fact is that one and the same organism 

 can live at one time as a plant, at another as an animal. Some 

 of the Chrysomonads have chlorophyll and live like green plants 

 in the sunlight, but when the conditions are not favourable for 

 this, they can capture and devour other organisms after the 

 manner of animals. The common Eitglena exhibits under 

 ordinary circumstances the metabolism of a green plant, but 

 when kept in the dark it is stated to be able to maintain its 

 existence as a saprophyte. Nothing shows more clearly than 

 these facts that the difference between plant and animal is, at 

 its first appearance, only a distinction based upon a habit of life, 

 a difference of degree and not of kind. If a Protist organism 

 can be at one time a j^lant, at another an animal, according to 

 circumstances, it is clearly impossible to use the distinction 

 between plant and animal for the purpose of subdividing the 

 Protista into two principal groups. 



Whatever the primary and original form of life may have 

 been on this we can but put forward speculations it must 

 have evolved in many different directions, and developed various 

 modes of vital activity. There is no reason whatever to suppose 

 that the possible methods of metabolism in living beings is limited 

 necessarily to two, three, or even a dozen or any other number. 

 As a matter of fact we find among Bacteria types of metabolism 

 which it is impossible to bring under the current limited classifica- 

 tions ; organisms, for instance, Avhich can fix free nitrogen, a 

 thing unknown in any green plant, saprophyte, or animal, but 

 quite as remarkable in itself as the fixation and decomposition 

 of carbonic acid gas by green plants. Moreover, distinctions 

 based upon metabolism and habit of life are quite unsuitable 



