844 ORIGIN OF SPECIES. 



further variations, because they are never prejudicial, but often on the contrary 

 advantageous for the purposes of adaptation. 



The perfect mode in which morphological characters are inherited 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 vital 

 conditions of the descendants, 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 vigo- 

 rously as to supplant the endosperm, w^hile it becomes itself filled wdth 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, w^hile 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 gradually became accustomed 

 to take up the assimilated food-materials of other plants or their available products 

 of 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 little substance as possible was em- 

 ployed 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 which were produced from an older progenitor, 

 and became in the course of time further difi"erentiated ; an order includes all the 

 genera which are descended with 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; and 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 

 classes and groups described at length in Book II, might be represented by lines, 

 which should express their actual affinity to one another ; and the system of diverg- 

 ing lines which would thus be obtained might be compared to an irregular system 

 of branching. In a plan of this kind we should proceed, starting from the lowest 

 Algae, along a number of lines of descent towards the various and more highly 

 developed classes of Algae. From the Siphonese a branch would shoot, beginning 

 with the Phycomycetes, itself branching copiously, and leading to the various forms 

 of Fungi. From a higher section of Algae another line would branch out which 

 would represent the Characeae; and in its neighbourhood another would be given 

 off which, splitting into two twigs, the Hepaticae and Mosses, would represent the 

 Muscineae. From the same neighbourhood another Hne would start which would 



