PRESIDENTIAL ADDRESS. 461 



There now remains only one point of general interest in the evolution of 

 the ceU to which brief reference must be made, namely, the divergence of 

 animal and vegetable cells. Not being a botanist, I desire to approach this 

 question with all caution ; but as a protozoologist it seems to me clearly indicated 

 that the typical green plant-cell took origin amongst the Flagellata, in that 

 some members of this group of Protozoa acquired the peculiar chromatophores 

 which enabled them to abandon the holozoic or animal mode of life in exchange 

 for a vegetative mode of nutrition by means of chlorophyll-corpuscles. It is 

 well known that many of these creatures combine the possession of chlorophyll 

 with an open, functional mouth and digestive vacuoles, and can live either in 

 the manner of plants or of animals indifferently or as determined by circum- 

 stances. It would be interesting to know exactly what these chromatophores, 

 at their first appearance, represent ; whether they are true cell-organs, or 

 whether, as some authorities have suggested, they originated as symbiotic 

 intruding organisms, primitively independent. I do not feel competent to 

 discuss this problem. I would only remark here, that if the green plant-cell 

 first arose amongst the Flagellata, then the distinction between plant and 

 animal (that is, green plant and animal) is not so fundamental a divergence 

 in the series of living beings as is popularly supposed, but is one which did not 

 come into being until the evolution of organisms had reached a relatively 

 advanced stage, that, namely, of the true nucleated cell. 



I have confined myself in this address to the evolution of the cell as this 

 organism is seen in its typical form in the bodies of the multicellular organisms, 

 starting from the simplest conceivable type of living being, so far as present 

 knowledge enables us to conceive it. But there is not the slightest reason to 

 suppose that the evolution of the Protista took place only in the direction of 

 the typical cell of the cytologist. Besides the main current leading up to the 

 typical cell there were certainly other currents tending in other directions and 

 leading to types of structure very unlike the cells composing the bodies of 

 multicellular organisms. It is impossible that I should do more here than 

 indicate some of the divergent lines of evolution, and I will confine myself to 

 those seen in the Protozoa. 



Taking as the starting-point and simplest conditioni in the Protozoa a simple 

 cell or protocyte, in which the body consists of a small mass of cytoplasm con- 

 taining a nucleus, with or without chromidia in addition, an early specialisation 

 of this must have been what I may term the plasmodial condition, typical of 

 Rhizopods, in which the cytoplasm increased enormously to form relatively 

 large masses. The nucleus meanwhile either remains single and grows very 

 large or, more usually, a great number of nuclei of moderate size are formed. 

 From this large plasmodial type is to be derived the foraminiferal type, 

 characterised by the creeping habit of life, and probably also the radiolarian 

 type, specialised for the floating pelagic habit. Both foraminiferal and radio- 

 larian types are characterised by an excessive development and elaboration of 

 skeletal structures, and the geological record proves that these two types of 

 organisms attained to a high degree of specialisation and diversity of form 

 and structure at a very early period." The Mycetozoa exemplify another 

 development of the creeping plasmodial type adapted to a semi-terrestrial mode 

 of life. 



In the Mastigophora the body generally remains small, while developing 

 organs of locomotion and food-capture in the form of the characteristic flagella. 

 In this class there is a strong tendency to colony-formation brought about by 

 incomplete separation of sister-individuals produced in the ordinary process of 

 reproduction by binary fission. The so-called colonies (they would better be 

 termed families) show a most significant tendency to individualisation, often 

 accompanied by physiological and morphological specialisation of the component 

 flagellate individuals. 



As an offshoot, probably, from ancestors of the Mastigophoran type arose 

 the Infusoria, the Ciliata and their allies, representing by far the most highly- 

 organised unicellular type of living being. No cell in the bodies of the 

 Metazoa attains to such a complication of structure as that exhibited by many 



-' For Foraminifera see especially Heron-Allen, Phil. Trans. (B). vol 20G 

 n915), p. 239. i ^ \ I 



