THE ANGIOSPERMAE 1253 



regular occurrence. For the vast majority pollination means cross-pollina- 

 tion, that is the conveyance of pollen from other flowers, either of the same 

 plant or of another plant of the same species. For this, three natural agencies 

 are available: wind, animals and water, and all are used to a greater or less 

 extent. The Gymnospermae, as we have described in Volume I, depend 

 on wind, with the doubtful exception of Wehvitschia. With this in mind 

 we might conclude that this is the primitive method and therefore the one 

 likely to prevail among primitive Angiosperms. This may possibly be true, 

 but it would be illogical to argue that flowers which are wind-pollinated are 

 therefore primitive. Our attitude will depend on what we believe about the 

 evolution of the Angiosperms. If we accept the theory proposed by Wett- 

 stein and elaborated by Engler, that the most primitive flowers are small, 

 achlamydeous and generally unisexual, like those of the Gnetales, then we 

 will agree that wind-pollination is the primitive method in the Angiosperms. 

 A contrary view is, however, possible, namely that the evolution of angio- 

 spermy and the development of insect pollination were closely linked. 



The geological record shows that the Angiosperms assumed their 

 present position of dominance rather rapidly and that an astonishing multi- 

 plication of genera took place within a comparatively short time. It is at 

 least plausible that this rapid evolution went hand-in-hand with the rapid 

 evolution of pollinating insects and that it was on this connection that the 

 biological success of the state of angiospermy depended. Not many of the 

 orders of Insects are ancient. The Neuroptera and Orthoptera can be traced 

 back to the Palaeozoic era; the Coleoptera were the principal insect order 

 during the Mesozoic, with Diptera and Hymenoptera as relatively minor 

 groups. The Lepidoptera and the bees, wasps and Syrphids only appear at 

 the beginning of the Tertiary. It seems reasonable to conclude that the 

 flowers and their pollinators were evolving side by side during the Tertiary 

 period and that this accounts for the complex yet harmonious balance that 

 exists between the structure of so many flowers and the insects which pol- 

 linate them. 



There is a general, though not universal, relationship between insect- 

 pollination and the presence of a coloured perianth. The Ranunculaceae, 

 for example, though having for the most part unspecialized flowers, are 

 insect-pollinated and have conspicuous perianths. Genera like Ranunculus, 

 Caltha and Ficaria have coloured perianths, but the flowers are regular 

 and open and ofl^er nectar freely to almost any visitor. In the same family, 

 on the other hand, genera like Delphinium and Aconitum, also with well- 

 developed perianths, are zygomorphic and specialized to the visits of only 

 humble bees and in the latter flower, to only one species of humble bee, 

 Thalictruni, which is only conspicuous from its abundance of coloured 

 stamens, is visited for the sake of its pollen. Within a single fairly primitive 

 family there is, therefore, a considerable range of flower-pollinator rela- 

 tionships which suggests that in the early phases of floral evolution many 

 diverse relationships may have sprung up, to form the starting-points of 

 distinct lines in floral evolution. 



