December 21, 1900.] 



SCIENCE. 



941 



of Protozoa, and the inevitable conclusion 

 from such study is that our schema of the 

 kinds of reproduction needs reforming. 



In common usage we distinguish the 

 sexual multiplication of animals, in which 

 the formation of the new individual is pre- 

 ceded by an act of fertilization, from the 

 asexual multiplication which takes place 

 without fertilization. The existence of 

 parthenogenesis makes such a distinction 

 difficult to maintain. We can no longer 

 consider this phenomenon as the ' mono- 

 gonous' method of reproduction, since the 

 typical cases, which are found in the Ar- 

 thropods, have apparently been derived 

 from sexual reproduction by the loss of fer- 

 tilization. The same is probably true of 

 what occurs in the Sporocysts and Eedige of 

 the Trematodes. In order to overcome this 

 difficulty and place parthenogenesis where 

 it naturally belongs, I would propose for 

 sexual reproduction the term reproduction 

 by means of germ cells. But I would not stop 

 with this alone, for my study of Protozoa 

 has developed in my mind a strong convic- 

 tion that our whole view of reproduction 

 needs a radical reformation. 



Eeproduction by the asexual method 

 alone was formerly considered an important 

 characteristic of the Protozoa, but this 

 breaks down entirely in the face of the in- 

 creasing number of observed cases in which 

 true fertilization occurs. From the obser- 

 vations recorded for Ciliates, many Flagel- 

 lates, Rhizopods of the most widely different 

 orders and numerous Sporozoa, I believe 

 that fei'tilization occurs in all Protozoa, and 

 that its rarity and the difficulty of demon- 

 stration are the only reasons its general 

 occurrence has not hitherto been observed. 



A still further objection to the term sexual 

 reproduction arises from the fact that we 

 often, if not generally, fail to discover any causal 

 relation between fertilization and reproduction. 

 The actual reproduction of the Protozoa is 

 accomplished either by their division into 



two or more individuals of equal size or by 

 the pinching off of smaller daughter ani- 

 mals from a larger mother. Now, before 

 we have a right to speak of reproduction 

 as ' sexual,' we must show that fertiliza- 

 tion exercises a determinative influence 

 upon its course. This might be proved by 

 showing that fertilization hastened repro- 

 duction or by showing that certain kinds 

 of reproduction occurred only in connec- 

 tion with fertilization. Any such determi- 

 native influence is now positively excluded 

 in a large majority of cases. 



In the ciliated Infusoria, where fertiliza- 

 tion in the Protozoa was first recognized as 

 such and has been most carefully described, 

 conjugation is not the forerunner but the 

 after result of active reproduction. Conju- 

 gation is even a hindrance to the multipli- 

 cation of Infusoria, because the necessary 

 reconstruction of the nucleus often occupies 

 many days, that are lost for the purpose 

 of reproduction. The power of division 

 of an Infusorian which has just finished 

 conjugation is, if anything, less than before, 

 and is never increased. In like manner, 

 the power of division in an Infusorian is 

 not decreased as it approaches the time 

 of conjugation, for when two individuals 

 are separated before the actual nuclear ex- 

 change has begun, they will divide even 

 more actively than animals after the conju- 

 gation has been normally ended. In fine, 

 one comes to the conclusiou that the advent 

 of conjugation in Infusorian cultures is not 

 a favorable sign for their further increase. 



Beyond any doubt, fertilization causes a pause 

 in the multiplication of many Flagellates and 

 Rhizopiods. Volvox when fertilized yields 

 resting spores, which will only develop after 

 a long period, during which they have been 

 frozen or dried. 



The same thing is observed after the con- 

 jugation of Algse, with which Volvox is quite 

 rightly placed by most investigators. 



The cyst of Actinophrys sol, which arises 



