THALLOPHYTA 



375 



encysted state of Euglena suggests a possible mode of initiation of the 

 encysted construction characteristic of Plant-Forms (compare Chapter 

 IX.). 



The saprophytic mode of hfe in Euglena — or parasitic, as it is m 

 many other Flagellates, which are then independent of light and 

 chlorophyll — suggests a distinctively animal existence. Here motility 

 is retained, and encystment appears only as an occasional incident. 

 This behaviour of Euglena, an organism in which sexuality does not 

 exist, or at least has not been observed, indicates that the segregation 

 of Animals and Plants may have antedated sexuality. But such ideas 

 must not be taken for more than they are worth, since they raise 

 questions which cannot be definitely answered. Nevertheless they 



Fig. 313. 

 Successive stages of fission of Eiigh-na : semi -diagrammatic. 



are worthy of consideration as giving a point of view which will 

 have its value in directing the study of the lower organisms, whether 

 of the Animal or of the Vegetable Kingdoms. 



The instability of nutritional method in Euglena — and especially 

 its mixed nutrition, partly photo-synthetic and partly saprophytic, as 

 it grows strongly in foul water — finds its parallel in the mixed nutrition 

 of many land-living Plants. It seems probable that irregular nutrition 

 has been widespread, from very low forms such as Euglena to the 

 highest Flowering Plants. At various points in the series the de- 

 pendence upon physiological supply other than by photo-synthesis 

 may have been accentuated. The parasitic Seed-Plants, such as Viscum 

 or Cuscuta, and the saprophytes, such as Neottia or Monoiropa, are 

 cases where it was adopted relatively late, in forms with their character 

 already stamped as Seed-Plants. The various groups of Fungi are 

 cases where the physiological dependence was established early, but 

 after the encysted state had been definitely adopted. A similar 

 segregation, but earlier still, with absence of encystment, would account 

 for the establishment of the Animal Kingdom. 



