General.

The origin and subsequent history of the New Zealand flora is, in great measure, a matter of speculation merely, for the material on which conclusions are to be based is in no small degree unsatisfactory and insufficient. In the first place, the all-important statistics as to floristic elements are, of necessity, drawn from existing floras, although the composition of these must be very different from those of the same areas in Tertiary times. It is also assumed, in phytogeographical writings generally, that the absence of a species from any area means that the species in question was never there, a supposition quite at variance with what is happening at present, let alone fossil records. In New Zealand, species can be seen in process of extinction, as in the case of Podocarpus spicatus in Stewart Island and the many relict species of the main islands. The family Podocarpaceae was formerly in the Chathams, as evidenced by pollen found in the peat (Erdtman, O., G., E. 1924: 679:70), the pollen being both of Dacrydium and Podocarpus type, yet there are now no conifers in the flora. Mida, until a few years ago, occurred on Juan Fernandez but the trees have all died and now it is a New Zealand endemic genus. Can it be possible that Xeronema has always been confined to New Caledonia and one tiny New Zealand islet'. But, it is not necessary to stress the ever-present dying out and coming-in of the members of a flora from families to jordanons. Again, although a genus has its richest development in some particular area, it by no means follows that such is the original centre of its distribution. For instance, in the large genera Hebe and Celmisia most of the species show signs of youth, and no phytogeographer would look for the origin of the closely-related Veronica in the former or for that of the equally closely-related Aster in the latter.

The matter of ancient land-connections, where there are now profound ocean-depths, is the burning question in New Zealand biogeography. But, here again, the ground is most insecure, and one can only say that the question of great changes in the relations of land and water is one on which there is about equal evidence for and against. Geology can make no definite pronouncement, and the matter, at present, rests solely on the facts of organic distribution and whether certain critical examples can be page 419clearly explained without the assumption of "land-bridges". To some this biological evidence is all-conclusive, especially from the zoological standpoint; Hedley for instance (1899: 393) going so far as to ignore the testimony of ocean-depths and construct a hypothetical land-area on biological considerations alone. Others, again will not allow land-connections at any price. With regard to plants, it is generally assumed, that there is a possibility of their being conveyed by wind, or even birds, across wide stretches of ocean, especially in the case of sporiferous species. But even with these latter, the spores of a plant, inhabiting only a windless forest-interior, could never be the sport of the wind. The case of Hymenophyllum ferrugineum of Juan Fernandez, Chile and New Zealand is hard to explain on the supposition of wind-carriage and equally difficult is that of H. Malingii, a pseudo-epiphyte, of quite local occurrence, on the dead parts of trunks of certain Podocarpaceae or Cupressaceae in New Zealand and Tasmania, but absent on neighbouring dicotylous trees. Were spores as readily carried by the wind as is supposed, there should be no special fern-floras, which is not the case; nor should the endemic Polypodium novae-zelandiae be confined to one portion of North Island, since it ascends to the subalpine belt. Regarding the seed-plants, the important evidence already given concerning the distribution of alien species in New Zealand, equipped in every way for travel and ecesis, and their relation to the primeval vegetation, shows how exceedingly difficult it is for a plant to gain entrance into a virgin plant-formation, also it has been seen of how slight advantage for long-distance travel is the possession of flying apparatus or of fruits palatable for birds, and how it is not the species which move but the associations to which they belong. Even absolutely bare ground, perfectly suitable as a seed-bed, is only occupied by species from the immediate vicinity, as in the case of the new ground after the Tarawera eruption, river-beds with their seed-catching mat-plants, ground left bare by retreating glaciers and many ideal places for seed-germination made by the operations of man.

Granting that plants of all kinds can be transported over thousands of kilometres of ocean, a supposition taxing one's judgement to no small degree, there comes in the carriage of animals. Now with regard to various classes of such there is a striking subantarctic affinity and it is the question of how their carriage has come about which has aroused the chief biogeographical discussion, the matter of a land-bridge with the antarctic or subantarctic being the main point of contention. As for land-connection in the north, almost all who have considered the subject, as will be seen further on, are in its favour. This question of the incoming of the sub-antarctic element of the New Zealand fauna is gone into at considerable detail by Hutton, Chilton, Benham and others²; here only a few cases are noted. But first it must be pointed out, that although it might be possible, though extremely difficult, to suggest a plausible explanation for every case on the supposition of ocean-transit, yet that such could apply to the organisms as a whole is a totally different matter.

Just as the botanical evidence of the last chapter and the zoological, of which the above is altogether incomplete, make out a strong case for a Subantarctic or Antarctic "land-bridge", so does the great depth of the ocean to the south and east of the New Zealand continental shelf shake one belief in the possibility of such connection. It is true that Captain Davies in Mawson's ship, the Aurora, discovered a small area of comperatively shallow water to the South of Tasmania, but he likewise demonstrated the presence of very deep sea between Macquarie and the Lord Auckland Islands. Further, as seen from the geographical chapter, New Zealand is surrounded by a fairly shallow sea, which to east, west and south suddenly sinks to a profound depth. Obviously, this shallow sea denotes an ancient land-surface, but the sudden drop affords strong evidence that deep water has existed, as at present, for an extremely long period. On the other hand, there may have been a long-continued earth-movement to which the present ocean-depth is due. However, the matter is one of mere speculation, and in the light of our present knowledge only, a belief or disbelief in land-connection rests solely on the belief in the possibility or impossibility of the plants and animals having been able to cross the vast stretch of ocean by means of wind-or bird-carriage alone. The difficulty of believing in this lengthy transoceanic transit is so great that I must declare for the problematical "bridge", but this must have existed at a time antecedent to the advent of mammals on the connected area.

Finally, comes the question of multiple origins. This is rarely seriously advocated at the present time, but, strange to say, in pre-Darwinian days, is was a common belief, though there was not a shred of evidence to show how such a phenomenon could take place. But in the present state of knowledge, there seems to me no reason, according to any accepted theory of evolution, and especially to the doctrine of mutation, why polygenesis should no take place occasionally. It would, of course, occur most frequently with regard to families and genera, and specific polygenesis might be a rare occurrence. But, in biogeographical discussions the idea of polygenesis will never be popular, for were it accepted such discussions would be futile.