10 



SEX AND HEREDITY 



few examples will be described illustrating the probable 

 course of its Evolution. 



The first example is one of those lowly organisms 

 which it is difficult to rank definitely either as an Animal 

 or as a Plant ; for it shares the qualities of both. It is 

 Euglena, one of the Flagellates, which in summer is 

 commonly found colouring the foul water draining from 

 manure heaps a vivid green. If a drop of this water be 

 examined under the microscope, many free-swimming, 

 pear-shaped bodies will be seen, propelled each by a 



single lashing flagellum 

 (Fig. 3). Each one is a 

 separate individual, and 

 contains within its little 

 naked mass of protoplasm 

 a nucleus, and several 

 green bodies to which its 

 colour is due. There is 

 also a bright red eye- spot 

 close to the base of the 

 flagellum or cilium. These 

 individuals multiply by fission or cleavage of the motile 

 cell to form two equal parts, each of which is a new indi- 

 vidual (Fig. 4). Here then is an increase in number 

 without any sexual process. The organism is motile in 

 water, a characteristic shared by many very simple 

 Animals. There is, however, a second phase of its life 

 which it enters when the circumstances are unfavourable. 

 The creature ceases to move, and surrounds itself with a 

 cell-wall. It becomes encysted (Fig. 3, D). But later, 

 when the conditions are favourable, it reverts to the 

 motile state, its protoplasm dividing, and escaping 

 from the ruptured cyst. In its encysted state Euglena 



FIG. 4. 



Successive stages of fission in Euglena : semi- 

 diagrammatic. 



