VARIOUS THEOEIES. 599 



the microscope, show a greater complexity of structure of their constituent cells 

 than do many Flowering Plants; and, should especial importance be attached to this 

 character, Diatoms and Desmids must be regarded as more highly organized than 

 many small annual Composites. The idea of progressive development implies a 

 recognition of that species of plant which is most highly developed and which 

 stands upon the apex of the pyramid, or, at any rate, of the group of plants which 

 has already reached the furthest point — is it the Aristolochiaceae, Cannaceae, Mag- 

 noliacese, the Orchids, the Composites, the Ranunculaceee, the Papilionacese, or the 

 Pomegranates? Any one who has studied carefully the structure of these plants 

 knows well that it is impossible to make an estimate of this kind. In a book 

 of Botany one group must be treated first and another last, but this does not 

 necessarily imply that the last is the most highly developed; indeed the various 

 writers of systematic works begin and end with the most various groups. Like the 

 theory of adaptability, that of progressive transformations from inherent forces fails 

 to give us a reasonable explanation of the variations which plants have undergone 

 in process of time. 



A third theory, based on the observations of modern times, is as follows: That 

 variations of form in the offspring arise through crossing, from the union of two 

 dissimilar protoplasts. This theory, based on the union of unlike forms, has been 

 fully sketched out in the last chapter. It assumes the existence in former times of 

 a vegetation rich in forms — an assumption amply justified by the fossil remains 

 which have been preserved. New forms arose, not by a progressive development 

 such as has been alluded to, but by a transformation or metamorphosis of those 

 already in existence. It was from the union of existent types that incipient new 

 species were produced. By the periodic recurrence of changes in climatic con- 

 ditions the areas of plant-distribution have received continual displacements, and it 

 was then that these incipient species or varieties were put to the test. Those well- 

 suited to the fresh conditions settled down into new species. They replaced their 

 less well-adapted ancestors in the plant-community, and they played the same part 

 as these had formerly done. A change indeed is brought about; but not (on the 

 lines of the theory of adaptability) as a direct result of climatic influences, nor from 

 an inherent tendency to progressive development. It arises rather from a change 

 in the specific constitution of the protoplasm in consequence of the crossing of 

 unlike forms. In basing the transformation of species on a crossing of this nature 

 we are relieved the necessity of picturing lacunae in a vegetation as a result of 

 climatic changes, or of any serious disturbance of the inter-relations of its various 

 component forms. Bacteria and Moulds, Mosses and Lichens, Ferns, Grasses, Palms, 

 and Coniferous Trees, have all of them a special function to fulfil in the great 

 community of plants, and they are to a certain degree dependent on one another. 

 Were one removed the whole would be affected, and it might well happen, did a 

 given group come to speedy extinction, that the whole community of plants might 

 suffer. But in every group at all times and in all places a reserve of new forms 

 continually arises by crossing, so that this danger is averted. With climatic 



