SECT. I MORPHOLOGY 169 



of former geological periods, it is safe to conclude that such Conifers 

 as Thuja, Biota, and the various Junipers, that now have scale-like 

 compressed leaves, have been derived from Conifers with needle- 

 shaped leaves. This conclusion is confirmed by the fact that on the 

 young plants of the scaly-leaved Conifers ty25ical needle-shaped leaves 

 are at first developed. The modified leaf forms do not make their 

 appearance until the so-called JUVENILE form has attained a certain 

 age, while in some Junipers needle-shaped leaves are retained through- 

 out their whole existence. Even still more instructive are the 

 Australian Acacias, whose leaf-stalks become modified, as phyllodes 

 (p. 45), to perform the functions of the reduced leaf -blades. The 

 demonstration of such an assertion is furnished by a germinating 

 plant of Acacia pycnantha (Fig. 174), in which the first leaves are 

 simply pinnate, and the succeeding leaves bipinnate. In the next 

 leaves, although still compound, the leaf-blades are noticeably reduced, 

 while the leaf-stalks have become somewhat expanded in a vertical 

 direction. At length, leaves are jiroduced which possess only broad, 

 flattened leaf-stalks. As many other species of this genus are pro- 

 vided only with bipinnate leaves, it is permissible on such phylogenetic 

 grounds to conclude that the Australian Acacias have lost their leaf- 

 blades in comparatively recent times, and have, in their stead, 

 developed the much more resistant phyllodes as being better adapted 

 to withstand the Australian climate. The appearance, accordingly, 

 of the phyllodes at so late a stage in the ontogenetic development of 

 those Acacias is in conformity with their recent origin. Lathyms 

 aphaca (p. 45), the leaves of which in the mature plant are transformed 

 into tendrils, has the first leaves of the seedling provided with 

 leaflets. It may, in like manner, be shown that in the case of 

 plants Avith similarly modified leaf forms, the metamorphosis of the 

 leaves does not take place until after the cotyledons and the first 

 foliage leaves have been developed, and it is then usually eff'ected 

 by degrees (^^''). 



Structural Deviations (^^^) 



Plants, even of the same species, never exactly resemble each 

 other. Every individual organism has its own peculiar characteristics 

 by Avhich it may be distinguished from every other of the same species. 

 To a certain extent these characters may be due to atavism, or the 

 reappearance of previous ancestral qualities. Most individual devia- 

 tions belong, however, to the so-called FLUCTUATING VAKIATIONS. 

 These occur in all species, and may be compared to the excursions of a 

 pendulum to either side of its position of equilibrium. More consider- 

 able departures from the normal for the species may be inherited. The 

 importance of these in the origin of species was already clearly recog- 

 nised by Charles Darwin. They have been called mutations by 



