260 THE AMERICAN NATURALIST [Vol. LVI 



loss, so that there is no distinotion between parent and 

 offspring. There is no death and thus it is that Weis- 

 mann and others have spoken of the ' ' immortality of the 

 Protozoa." It is certainly significant, in this connection, 

 that among the Foraminifera 56 per cent, of the genera 

 were found to be persistent and many were found to 

 exhibit tremendous persistence, ranging from the Ordo- 

 vician, Silurian, Carboniferous and Triassic to recent 

 times, and that even species (see Ruedemann, op. cit. 

 p. 126) are known to extend from Silurian, Devonian, 

 Carboniferous and Triassic times to the present. These 

 forms the writer designated as actual " immortal types " 

 in contrast to the theoretically immortal protozoans of 

 Weismann. 



There occurs, however, among the protozoans besides 

 this asexual mode of reproduction a group of processes 

 that are clearly the primitive beginnings of fertilization. 

 In these forms of conjugation different stages may be dis- 

 tinguished, viz., the mere congregation of cells in groups 

 without visible exchange of plasms (cytotropy) ; the ex- 

 change of substance taking place only through osmotic 

 processes; further conjugation, where real fusion of 

 plasmas occurs but the cell-nuclei remain separate (plas- 

 togamy) ; and finally such modes of conjugation, where 

 also nuclear fusion of the conjugating cells takes place 

 (karyogamy) ; and here again, the pairing cells may be 

 either similar in size (isogamy), or even markedly dis- 

 similar in size (anisogamy). 



It is, however, to be remembered th^at the usual re- 

 productive process among protozoans is simple fusion 

 of ordinary vegetative cells and conjugation as a rule 

 occurs at rare intervals in most forms, often only when 

 unfavorable conditions arise, or as Maupas' experiments 

 indicate, the individuals in the course of numerous suc- 

 cessive asexual generations grow old. 



(h) Reproduction by Budding. — This mode of asexual 

 reproduction differs from that of division originally in the 

 protozoans merely in the different sizes of the daughter- 



