GENESIS. 229 



supply of food be artificially maiutained, tJie agamogeiiesis 

 continues through the winter. Nay more — it not only, under 

 these conditions, continues through one winter, but i^ has 

 been known to continue for four successive years ; some 

 forty or fifty sexless generations being thus produced. And, 

 those who have investigated the matter, see no reason to 

 doubt the indefinite continuance of this agamogenetic mul- 

 tiplication, so long as the external requirements are duly 

 met. Evidence of another kind, which points very 



distinctly to the same conclusion, is furnished by the hetero- 

 genesis of the Daphnia — a small crustacean commonly Imown 

 as the Water-flea, which inhabits ponds and ditches. From 

 the nature of its habitat, this little creature is exposed to very 

 variable conditions. Besides being frozen up in winter, the 

 small bodies of water in which it lives, are often unduly 

 heated by the summer sun, or dried up by continued drought. 

 The circumstances favourable to the Daphnia's life and 

 growth, being thus liable to interruptions which, in our cli- 

 mate, have a regular irregularity of recurrence ; we may, in 

 conformity with the hypothesis, expect to find both that the 

 gamogenesis recurs along with evidence of declining nutri- 

 tion, and that its recurrence is very variable. This we do 

 find. From Mr Lubbock's paper on the Daphnia in the 

 " Philosophical Transactions " for 1857, and from further 

 information which he has been good enough to furnish me, 

 the following general facts are deducible : — First, that in 

 each ovarium, along with the rudiments of agamic eggs, or 

 eggs which, if developed, produce young by true partheno- 

 genesis, there usually, if not always, exists the rudiment of 

 an ephippial agg ; which, from sundry evidences, is inferred 

 to be a sexual or gamic egg. Second, that according to cir- 

 cumstances, either agamogenesis or gamogenesis takes place ; 

 but that if the agamic eggs develop, the rudimentary gamic 

 egg disappears, or becoJies absorbed ; and conversely, if the 

 gamic egg develops, the agamic eggs disappear, or are ab- 

 sorbed by it. Third, that the brood of agamic eggs contained 



