714 INHERITANCE 



AGE CHANGES: SEXUAL IMMATURITY AND MATURITY 



Beginning with an individual that has recently conjugated, if the lines 

 of descent by vegetative reproduction are followed for great numbers 

 of generations, certain characteristics of the individuals are found gradu- 

 ally to alter. The offspring produced at different periods differ. In Uro- 

 leptus mohilh (Calkins, 1919) or in Paramecium hmsaria (Jennings, 

 1939), the individuals are at first sexually immature; they do not con- 

 jugate under any conditions. This continues for many generations of 

 vegetative multiplication. The offspring during this period are like the 

 parents in this respect. 



But after many generations have passed, the descendants gradually 

 become sexually mature. They now conjugate when mixed with indi- 

 viduals of different mating type. These descendants are thus different 

 in this respect from their earlier ancestors. In this period their own off- 

 spring inherit from them the mature condition. 



In Paramecium bursaria, and presumably in other species, the mature 

 condition comes on slowly and gradually. There is for many generations 

 partial maturity, in which the tendency to conjugate is but slight. The 

 tendency becomes stronger as generations pass, until full maturity is 

 reached. The period of full maturity lasts for a great number of genera- 

 tions, during which the mature condition is inherited in vegetative repro- 

 duction. Such periods of immaturity and maturity were described fifty 

 years ago by Maupas (1889) for a number of species of ciliates. In some 

 species, however, they hardly exist, or the period of immaturity if it 

 occurs at all is very short. Such is the situation in P. aurelia (Sonne- 

 born, 1936). 



At a late period in the life history, in some species the individuals are 

 found to become less vigorous as generations pass. They multiply less 

 rapidly, become "depressed," degenerate. Whether this is an additional 

 period in the life history, beyond the periods of immaturity and ma- 

 turity; whether, in other words, it is an age change, constituting a period 

 of senescence and final senility, or whether it is only a degenerate condi- 

 tion arising in consequence of living long under unfavorable conditions, 

 appears as yet unsettled. This period of decline will therefore be con- 

 sidered in the next section. 



In some of the Protozoa, particularly among parasitic forms, in dif- 



