THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 615 



theoretical considerations, and of bringing forward the various groups which have 

 from the beginning existed side by side, distinguishing them by the enumeration 

 of their peculiar characteristics, I am not blind to the enormous diflBculties of the 

 undertaking. Although Palaeontology, Morphology, and Physiology afford valuable 

 results, they are not enough, and neither of the three sciences gives sufficient data 

 for the complete solution of the problem. One of the greatest obstacles is the afore- 

 said incompleteness of the geological record. From the existing remains we may 

 indeed conclude on the whole that numerous groups stood side by side in the 

 mesozoic and palaeozoic periods, but the evidence of many groups which exist at 

 the present day without transitional forms is wanting, and when we assume their 

 existence we make use indeed of a justifiable hypothesis, but have no proof what- 

 ever. The danger, on the other hand, of establishing homologies from the similarity 

 of an organ which is observed in groups of species now living has already been 

 mentioned (see p. 609). Up to a certain point all organs which have similar work 

 to perform agree with one another. This agreement is the more pronounced the 

 greater the similarity of the conditions under which the organs have to do the 

 work. Species of very different groups which live under water exhibit many 

 characteristics in common; plants whose pollen-grains are transported by the wind 

 show a great agreement in the structure and position of the parts of the flower. 

 In the same way the form of flower- visiting insects necessitates a number of similar 

 characteristics in the flowers visited. For example, we might instance the sweep- 

 ing hairs on the style of the Proteacese and of the Composites, as well as certain 

 developments which are met with in the flowers of Aroids, which are visited by 

 small flies, and also in the Aristolochiacese. In spite of this consideration, however, 

 the similarity in the structure and form of organs, both of those serving for pro- 

 pagation and for nourishment and growth, must obviously be kept to the forefront; 

 similarity must always be an important factor in the limitation of groups. 



As we have in the preceding chapter established the fact that each species is 

 built up by protoplasm with a specific constitution, the question might be pro- 

 pounded whether each plant-group has not something in common in this respect. 

 Many observations argue differently for this view. It has been repeatedly stated 

 that the Moulds, Oscillatorie^, Sea-wracks, Stoneworts, &c. give off a scent which, 

 although it difiers according to the species, is yet very similar upon the whole, and 

 that one is justified in assuming a specific constitution of the protoplasm in each of 

 these groups on this account. Moreover, the scent which the Mosses exhale is 

 found in no other group of plants. The same is true of Ferns. The delicate fronds 

 of the tropical Filmy Ferns exhale the same peculiar scent as the larger Ferns of 

 our forests. The Coniferse, Umbelliferae, Labiateae, Leguminosse, and Cruciferae 

 exhibit similar conditions. Is it not also a striking phenomenon that the parasitic 

 Fungus Cronartium asclepiadeum should settle on Cynanchum Vincetoxicum, as 

 well as on Gentiana asclepiadea, i.e. upon two plants which the Botanist certainly 

 places in difierent families, but which he regards as belonging to the same alliance ? 

 To these facts many others might be added, especially with regard to the choice of 



