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 Hepaticae 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 

 peculiarities 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 Httle or no service to the new form, and therefore as little 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 



