Cytology

Frequently the members of a genus comprise one or more species which extend over a large area, whilst the remainder are of much more restricted distribution and occupy areas either in cluded in or overlapping the range of their more widespread con geners. Illustrations are furnished by the she-oaks of Australia (Casuarina), with the littoral C. equisetifolia extending from east Africa and Madagascar to the Society islands and Burma and the Philippines to Tasmania, within which area occur a num ber of other species. Again the genus Mercurialis includes the dog's mercury (M. perennis) and M. annua, which are of wide range, whilst the five or six other species extend over compara tively small areas in the Mediterranean region and eastern Asia. It is probable that in such genera the species of more local occur rence are derivatives of the wide-ranging types, but it is also evident that were the widely dispersed parent types to die out the other species of restricted distribution would come to present the condition of discontinuous distribution which is a feature of many genera and families at the present day. The pipeworts (Eriocau lon) are chiefly found in tropical and sub-tropical regions, but E. septangulare is found in the eastern United States, the west of Scotland, and the west of Ireland. Of over 400 species of heath belonging to the genus Erica, the majority are concentrated in Cape Colony and the Mediterranean area. The Proteaceae and Cycadaceae, containing many very localized genera, now exhibit a discontinuous distribution in tropical and sub-tropical regions, but there is little doubt that the Cycadaceae extended over a large part of the world in Jurassic times and the Proteaceae were probably much more widely distributed in the Tertiary epoch than at the present day. Discontinuous distribution in the species may be the outcome of irregularities of dispersal, the result of extinction from parts of its former range, or even due to origin of the species in more than one locality.

Effects of Glaciation.

It is quite impossible to appreciate the present distribution of species in either North America or northern Europe without recognition of the profound changes consequent upon the last glacial epoch. During Cretaceous times there would appear to have been a remarkable uniformity in the character of the vegetation of the world, with warm temperate types as far north as Greenland. This uniformity was succeeded by a differentiation in the Tertiary period. On either side of the tropical belt there probably existed a zone of forest presenting a mixture of i temperate and subtropical types, southern types be ing present n the northern hemisphere and northern types, such as the true beeches and the common oak (Quercus Robur), being present in the southern hemisphere. Mrs. Clement Reid has admirably demonstrated, by a comparison of successive floras of the Pliocene period of Europe, the gradual disappearance of exotic types as the Pleistocene approached, probably accompany ing a diminution of temperature. It is due to the work of Heer on the arctic fossil flora, the writings of Asa Gray, Saporta and Hooker that the fossil floras of the past have been so ably util ized in the elucidation of the peculiarities of the distribution of the present. The holarctic distribution of a large assemblage of

species prior to the glacial refrigeration is the clue to the resem blances between the American and east Asiatic floras. Of these migrants, driven southwards by the increasing cold, relics remain in the genus Sequoia, now only represented by two north-west American species, in Ginkgo biloba, confined to western China, in the American genus Liriodendron and the Chinese Glyptostrobus. The magnolias no longer occur wild in Europe as they did in the past, but still survive both in Asia and America. Of the genera cited Depape has recorded Pliocene remains from the valley of the Rhone of Sequoia sempervirens, Liriodendron tulipif era, Ginkgo biloba, and Glyptostrobus heterophyllus. The elimination of these and many others from the European flora is probably to be attributed mainly to the presence of natural barriers against the southward retreat, such as the Mediterranean, whereas in Asia and America there was a continuous land bridge to a climatic sanctuary, the temporary home of some, the permanent retreat of others.

During the Ice age itself the flora consisted of definitely arctic types and a considerable number of other hardy species which survived the cold in the areas, probably of considerable extent, which were not actually covered by ice. Judged by the frequency of the remains in sub-fossil deposits of the glacial period in Europe the commonest species were the mountain dryas (Dryas octopetala), the dwarf birch (Betula nana) and various willows of mat-like habit, such as Salix reticulate, S. polaris, etc. It is note worthy that the species cited and a number of others prevalent at that period are now most characteristic of arctic regions but also occur in warmer climates on what may be termed arctic islands, namely, the summits of the higher mountains, a discontin uous distribution that can be attributed to survival from glacial times. With the final retreat of the ice, which in northern Europe probably took place somewhere about 14,000 years ago, the recol onization of the glaciated areas by the less hardy species began.

The colonization thus begun has continued ever since. The fact that Ireland possesses but two-thirds of the vascular plants of Britain whilst the flora of England, though richer than that of Scotland, is less varied than that of the neighbouring areas of France emphasizes the importance of the facilities for dispersal of fruits and seeds in determining the distribution of species. The rapid spread of the Canadian pondweed (Elodea canadensis) from Market Harborough, where it first appeared in England in 1847, of Veronica persica, or more recently of the rayless may weed (Matricaria suaveolens), a native of Oregon, all of which have become abundant over a great part of Britain; or, again, the spread of the prickly pear (Opuntia inermis) at the rate of a million acres a year in Australia and the choking of the water ways of Florida by the water hyacinth (Eichornia crassipes) all bear witness that area is not necessarily a criterion of age and that the process of colonization is still going on, a process in which the disturbing influence of the hand of man plays no small part.