586 SUMMARY OF CURRENT RESEARCHES RELATING TO 



(Jordans' " little species," Johannsen's " pure lines ") which differ inter se 

 in size, form and physiological qualities, but are in se constant. Within 

 each race individuals differ in size, as a result of peculiarities of growth, 

 nutrition, and environment. But the races all breed true to their mean 

 dimensions. Selection within the pure race is of no effect on the size. 

 Many other investigations bearing on variations and modifications and 

 their hereditary behaviour are dealt with. 



The sexual period is then discussed. Assortative mating or the 

 conjugation of like with like has been proved by Pearl in Paramecium 

 caudatum, but it does not seem to be general. Many experiments con- 

 tradict the view of Maupas that there is a " eugamic period" or a period 

 of " karyogamic maturity " in the life-cycle. E. Hertwig's idea that 

 the " hunger divisions " among starved Ciliates correspond to maturation 

 divisions is erroneous, for the Ciliate is a sexual individual and not 

 comparable to a germ-cell. The phenomena in a Ciliate which are 

 comparable with the two meiotic divisions in a metazoon are the micro- 

 nuclear divisions preceding fertilization. Many suggestions have been 

 made in regard to the conditions inducing conjugation — whether hunger 

 (Maupas), disturbed karyoplasmic ratio (R. Hertwig), changes in the 

 medium (Enriques), plentiful feeding followed by starvation, followed 

 by treatment with salts in medium concentration (Zweibaum), and so on. 

 As to the effects of conjugation, Maupas emphazised karyogamic rejuven- 

 ation, Biitschli and Calkins the counteracting of a slowed rate of division 

 and a condition of senility. But R. Hertwig and Jennings have shown 

 that this conclusion is not justified. Statements or implications that 

 conjugation in Infusorians results in increased multiplication or vitality 

 should disappear. Jennings concluded that conjugation results in 

 variation and in biparental inheritance, which sounds like simultane- 

 ously producing uniformity and diversity. Dobell takes the paradox 

 to mean that if two similar individuals of the same race conjugate, then 

 the progeny of both will differ from the original race, though the 

 progeny of one will resemble the progeny of the other in whatever 

 respects it- differs from the original race. Or, in other terms, a pair of 

 conjugants a x and a 2 , belonging to a race a, produce after conjugation 

 progeny, forming races b r and b 2 , differing from a, but resembling one 

 another in both being b. Both the fundamental conclusions reached by 

 Jennings appear to Dobell unproved. Recent researches on inter- 

 conjugation and cross-conjugation are discussed. 



Dobell believes that many of the problems now associated with 

 Ciliates are really dialectic, and do not exist in nature. Many are due 

 to the error of thinking of a Ciliate as a cell, instead of thinking of it 

 as a complete organism. It has been shown beyond all reasonable doubt 

 that under suitable conditions Ciliates are able to hive and multiply in 

 their own fashion, for an unlimited time, like all other organisms that 

 are well adapted to their environment. If " immortality " can be pre- 

 dicated of the Ciliates, it can also be predicated of all other organisms 

 in the same sense. It may be stated with considerable confidence, as a 

 concrete proposition, that conjugation in the Ciliates does not result in 

 rejuvenation, no matter whether a literal or metaphorical meaning be 

 attached to the word. 



