942 
ORIGIN OF SPECIES. 
The fact that members which are morphologically similar are adapted for the 
most various functions is explicable when we consider that the morphological 
features in the structure of plants are those which are most certainly transmitted 
unchanged to posterity, either because they have no direct relation to the struggle 
for existence, or because they have proved useful in the various relations of life ; 
as for example the differentiation into stem, root, leaves, &c., and into the different 
tissue-systems, by which the division of physiological labour and the acquisition of 
the most various properties useful for the struggle for existence are facilitated. The 
structure of the Thallophytes and of the Hepaticse shows that these morphological 
differentiations do not exist in the first or lowest forms of plants, but that they come 
gradually into existence ; but when once fully developed they are preserved in the 
course of further variations, because they are never prejudicial, but often on the 
contrary advantageous for the purposes of adaptation. 
The perfect heredity of morphological characters gives rise to a very remarkable 
phenomenon, the production of functionless members. It is obvious that hereditary 
pecuharities may have lost their use under the new conditions of life of the de- 
scendants, because the physiological requirements of the plant are supplied by other 
means, by fresh adaptations. Of this nature are, for example, the minute leaves 
on the root-like shoots of Psilotum, the formation of endosperm in the embryo-sac 
of many Dicotyledons whose embryo afterwards grows so vigorously as to supplant 
the endosperm, while it becomes itself filled with reserve food-materials which in 
other cases are stored up in the endosperm for the seedling. The most striking 
illustration however is the behaviour of parasites and saprophytes destitute of 
chlorophyll, which are found in various orders of plants, and the near allies of 
which form large green leaves containing chlorophyll, while these produce leaves 
similar in a morphological sense, but which are neither large nor green, and 
sometimes degenerated so as to have become obsolete. The explanation of this 
phenomenon is at once afforded by the theory of descent, viz. that the parasites 
and saprophytes which contain no chlorophyll are the transformed descendants of 
leafy ancestors which did form chlorophyll, but which have gradually become ac- 
customed to take up the assimilated food-materials of other plants or the available 
products of their decomposition ; and the more they did this the less needful did 
it become for the plants themselves to assimilate. The green leaves therefore 
became meaningless and ceased to form chlorophyll; but without chlorophyll the 
leaves were of little or no service to the new form, and therefore as Httle substance 
as possible was employed in their development, and they gradually degenerated. 
Looked at from the point of view of the theory of descent, the natural system 
of the classification of plants represents their blood-relationship to one another. 
A species consists of all the varieties which are descended from a common ancestral 
form ; a genus of all the species produced from an older progenitor, and which 
have become in the course of time further differentiated ; an order includes all the 
genera which have been derived by variation from a still older ancestral form ; and 
the first primitive form of all the orders comprised in a group belongs to a still 
older past ; finally there must have been a time when a primordial plant originated 
the whole series of development ; and this must have produced in its varying de- 
scendants the primitive types of all the later forms. The relationships of the various 
