290 THE INDUCTIONS OF BIOLOGY. 



special influences, is furnished by the heterogenesis of the 

 Daphnia a small crufetacean commonly known 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 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 declining physical prosperity 

 and that its recurrence is very variable. I use the expres- 

 sion " declining physical prosperity " advisedly ; since " de- 

 clining nutrition," as measured by supply of food, does not 

 cover all the conditions. This is shown by the experiments 

 of Weismann (abstracted for me by Mr. Cunningham) who 

 found that in various Daphnidece which bring forth resting 

 eggs, sexual and asexual reproduction go on simultaneously, 

 as well as separately, in the spring and summer: these 

 variable results being adapted to variable conditions. For not 

 only are these creatures liable to die from lack of food, from 

 the winter's cold, and from the drying up of their ditches, &c., 

 as well as from the over-heating of them, but during this 

 period of over-heating they are liable to die from that de- 

 oxygenation of the water which heat causes. Manifestly the 

 favourable and unfavourable conditions recurring in com- 

 binations that are rarely twice alike, cannot be met by any 

 regularly recurring form of heterogenesis; and it is interest- 

 ing to see how survival of the fittest has established a mixed 

 form. In the spring, as well as in the autumn, there is in 

 some cases a formation of resting or winter eggs; and 

 evidently these provide against the killing off of the whole 

 population by summer drought. Meanwhile, by ordinary 

 males and females ijiere is a production of summer eggs 

 adapted to meet the incident of drying up by drought and 



