382 THE NATURAL HISTORY REYIEW. 



we know, been said, even by so competent an observer as Mr. 

 Berkeley, that tliere is only a relation of analogy, and not one bf 

 homology between tbe two classes of plants.* Mr. Spencer evidently 

 is of a different opinion, for lie traces tbe constitution of the stem 

 up from those plants which present the earliest traces of such an 

 organ, to those in which it exists in greatest perfection. He shows 

 that in many Mosses, Hepaticse, etc., there is no real differentiation 

 between leaf and stem ; the thallus is the representative of both. He 

 further shows that in many of them, there is a separation of new 

 plants or buds, while in other cases these buds remain attached to 

 the parent, and become shoots, the lower part of the original frond 

 becoming thickened, and its rootlets increased in number, so as to 

 afford a sufficient supply of nourishment to the young shoot. If 

 this means of deriving more nourishment be not possible, then the 

 new bud will be thrown off as a distinct plant to gain its own liveli- 

 hood independently of the parent. The influence of Mr. Darwin's 

 theory on the mind of our author now shows itself, for says he with 

 much reason, " there will be extra growth of those rootlets which are 

 most serviceably placed," and " such a variation implying as it does 

 circumstances especially favourable to the growth of the plant, will 

 give to the plant extra chances of leaving descendants ; since the 

 area of frond supported by a given area of the soil, being greater 

 than in other individuals, there may be a greater production of 

 spores, and then, among the more numerous descendants thus 

 secured by it, the variation will give advantages to those in which it 

 recurs." If it become the habit of the primary frond to bear a 

 secondary frond from its midrib, this secondary frond, composed of 

 physiological units of the same kind, will inherit the habit ; and so a 

 tertiary frond will probably be followed by a quaternary one, and 

 so on ; each new frond necessitating, or being accompanied by an 

 increased thickening of the midrib of the lower fronds, and an 

 increased development of roots. And so ultimately we shall get a 

 series of fronds placed alternately on opposite sides of a continuous 

 axis, which increases in length, but very little transversely, unless, 

 indeed, the stem is to rise perpendicularly, and bear large foliage, 

 when the stem is proportionately thickened as well as lengthened. 

 As plants thrive best when most fully exposed to light and air, so, 

 argues Mr. Spencer, more Darwinii, natural selection will favour the 



♦ Berkeley, Introd to Cryptog. Botany, p. 32. 



