48 



THE EVOLUTION OF SEX. 



go to show, for some cases, that abundant moisture and nourish- 

 ment do tend to produce females. Some of JNIeehan's points 

 are extremely instructive. Thus old branches of conifers, 

 overgrown and shaded by younger ones, produce only male 

 inflorescence. Various botanists, quoted by He^-er, confirm 

 one another in the observation, that prothallia of ferns grown 

 in unfavourable nutritive conditions produce only antheridia 

 (male organs), and no archegonia or female organs. 



The botanical evidence, though by no means very strong, 

 certainly corroborates the general result that good nourishment 

 produces a preponderance of females. The contrast of the 

 sexes in our common dioecious plants is here very instructive. 

 Taking for instance the dog-mercury {Mercurialis pere?ims) of 

 any shady dell, or the day lychnis (L. diurna), so often hardly 

 less abundant on its sunnier slopes, experiments are still 

 certainly wanting with regard to given plants, as to what cir- 



i\Iale r.r.d Female Flowers of Pink Campion {Lychnis dhirnd). 



cumstances originally determined their sexual differences ; but 

 the fact of superior constitutional vegetativeness in the females 

 is here so peculiarly obvious, that it can hardly fail to arouse 

 a strong impression, that more or less advantageously nutritive 

 conditions, whether of the embryo or of the seedling, are suffi- 

 cient to account for the differences of sex. 



§ 2. Influence of Tempei-atiire. — In this connection not a 

 few writers have referred to an obser^-ation by Knight, which, 

 from its comparatively ancient date, perhaps deserves to be 

 recorded in his own words, if only to show the necessity of 



