FERTILIZATION IN PLANTS AND UNICELLULAR ORGANISMS 327 



reproduced sexually, for they still possess the sac which serves for 

 receiving spermatozoa, but this sac remains empty, for there are now 

 no males, at least in any habitat of the species known to us. To this 

 set belongs an inhabitant of stagnant water, LimimcUa hermanni, a 

 species of Crustacean which was found thirty years ago in hundreds, 

 all of the female sex, near Strassburg, and also many of the little 

 Ostracods {Cypris) which inhabit especially the muddy bottom of our 

 pools and marshes. I bred one of these {Cypris reptans) in numerous 

 aquaria for sixteen years, during which there were about eighty 

 generations, and throughout this time no male ever appeared, nor did 

 the sperm-sac of the female ever contain spermatozoa. The after- 

 effects of the ' rejuvenating ' power of an amphimixis supposed to have 

 taken place earlier must in this case have been enduring indeed ! 



For these reasons it seems to me useless to make comparisons 

 between the developmental cycle of unicellular organisms and the 

 ontogeny of multicellular organisms. Both processes have indeed 

 many points of resemblance — long series of cells, then interruption of 

 the divisions and the occurrence of amphimixis — so tliat we may quite 

 well speak of cyclic development in the physiological sense, in as 

 far as certain internal conditions periodically recur and compel the 

 organism to conjugation, but we must not suppose that there is more 

 in this than, for instance, in the ' cyclic development ' of Man, which 

 consists in the fact that he finds himself periodically impelled to take 

 food. The feeling of hunger which forces him to do so is the signal 

 which warns the organism that it is time to supply fresh combustible 

 material to the metabolism. In the same way, after a long series 

 of generations of Infusorians the necessity for conjugation arises; 

 the whole colony suffers an ' epidemic of conjugation,' and the animals 

 unite in pairs ; in the meantime we know not why, and must content 

 ourselves with formulating what is observable, that the nuclear 

 substances of two individuals are thereby mingled in each conjugate. 



Obviously the impulse to conjugation is a signal in the same 

 sense as the feeling of hunger is, and we know well from the higher 

 animals what a mighty influence it exerts, an influence hardly less 

 potent than that of hunger. In Schiller's words, ' Durch Hunger 

 und durch Liebe, erhalt sich dies Weltgetriebe.' 



We can see clearly enough why Nature should have given 

 animals the feeling of hunger, but the reason for the need of con- 

 jugation is not so plain ; we can only say in the meantime that it 

 must be of some value in maintaining the forms of life, for only that 

 which fulfils a purpose can be permanently established. 



I shall return later to the problem of the meaning of 'sexual 



