82 



OUTLINES OF EVOLUTIONARY BIOLOGY 



state of nature the chief if not the only means by which the 

 family can be kept from speedy extinction is conjugation, the 



FIG. 37. Life History of Copromonas. (From Bourne's " Comparative 

 Anatomy," after Dobell.) 



cv., contractile vacuole; cpJi., cell pharynx; cst., cell mouth; fv., food vacuole; 

 N., nucleus; R., reservoir; tr., flagellum. (For further explanation see text.) 



exhausted individuals approaching one another and finally uniting 

 in pairs. In this way they appear to become rejuvenated and 



average of about one division every fifteen hours and at the end of the period the 

 organisms were still in a perfectly normal condition. 



