ALTERNATION" OF GENERATIONS I 77 



The phyletic development of these two kinds of germ-plasm 

 would be very enigmatical if we were compelled to assume that 

 only a single unit of the germ-plasm is present in the nucleus of 

 the germ-cell. We have, however, made the reverse assump- 

 tion from the first, and it will be shown later on that a con- 

 sideration of sexual reproduction, or amphimixis, leads us to 

 assume that several, and in fact probably a large number of 

 units or ids must be contained in the germ-plasm of every 

 species which multiplies sexually. If now a reduction of the 

 determinants for the nauplius in the summer generation of 

 Leptodora were advantageous, it would have appeared, increased, 

 and become fixed in the course of generations by selection, and 

 an abbreviation of embryogeny would thus have resulted. This 

 would only have occurred gradually, so that at first the summer- 

 eggs would contain more reduced than unreduced ids only in the 

 case of a few individuals ; and if the original unabbreviated form 

 of embryogeny were of greater advantage to the winter genera- 

 tion, the determinants for the nauplius would not become lost or 

 modified in all the ids. but only in certain of them. A balance 

 of the two kinds of ids would finally take place from the struggle 

 between the modified ids, which were more advantageous in 

 summer, and the unmodified ones, which were of greater advan- 

 tage in winter, and this would result in the germ-plasm of the 

 species being composed of an equal number of modified and 

 unmodified ids ; these would alternately control the cell, so that 

 each would remain inactive and unalterable during a certain 

 number of generations, and become active during certain others. 



This regular alternation between definite periods of activity 

 and inactivity in the two kinds of germ-plasm can be directly 

 observed, for we can determine how many generations occur 

 which give rise to summer-eggs before one again appears in 

 which winter-eggs are produced. As I was able to prove a con- 

 siderable time ago, this regularity varies very much in different 

 species of Daphnidae, and stands in close relation to the mode 

 of life of the species. In those species which live in verv small 

 bodies of water which are liable to become rapidly dried up, the 

 formation of the two kinds of eggs alternates verv frequently ; 

 this is due to the fact that the extermination of the animals by the 

 sudden drying up of the pond is only prevented by the thick 

 shells by which tlie winter-eggs are surrounded. On the other 

 hand, all the species which live in large bodies of water, such as 



