530 MANUAL OF NATURAL HISTORY. 



of the object under investigation. But the present 

 amount of our knowledge, while allowing us gradu- 

 ally to approximate truth, does not enable us finally, 

 as yet, to arrive at the settlement of this important 

 question. In the lowest members of both kingdoms, 

 sexual distinctions are either wanting or extremely 

 obscure, and increase seems to be effected by a spon- 

 taneous multiplication of cells. So far, therefore, 

 both are alike, but here the resemblance ceases ; as 

 we advance upwards in the Zoological series, we 

 first find beings in whom the sexes are united, but 

 afterwards male and female are completely sepa- 

 rated, which is invariable in the higher animals. 

 Among vegetables, again, the existence of the two 

 sexes in distinct individuals is more general, rather 

 low down in the scale, while hermaphroditism 

 seems to be the normal condition of the most 

 advanced plants. A difference in the sources of 

 movements in these two elementary forms is highly 

 probable, and may be further established by ana- 

 logy with the more developed beings of either class. 

 The one seems more the result of internal causes, 

 originating, perhaps, in a kind of instinctive volition, 

 while the other would appear to depend rather on 

 external agents, and to be more allied to irritability. 

 Among Thallogens are ranked, first, the Lichens, 

 denizens of forest, of mountain, and of plain, ranging 

 from the torrid zone to frozen climes ; curious 

 little plants, their gracefully twisted and often sil- 

 very fronds at times giving a venerable appearance 

 to the trees whose trunks they frequently clothe, or 



