6] FOUNDATIONS OF MODERN DISTRIBUTIONS 167 



previously dissimilar types is even more improbable. Accordingly, 

 similarities across wide stretches of water are more popularly 

 explained by suppositions of previous proximity or even contiguity, 

 though, as we have already suggested, dispersal may be more effective 

 than is commonly supposed, and, conceivably, responsible for some 

 at least of the apparent anomalies of distribution. 



If it were accepted for Mesozoic and later times, the hypothesis 

 of continental separation and drift would make unnecessary, or at 

 all events less necessary, several of the following and other theories 

 that are of interest in plant geographical considerations — though it 

 may be confidently assumed that it will not be so accepted without 

 far more conclusive evidence than has yet been presented. Mean- 

 while it should be noted that this theory is supported by, or is in 

 accordance with, a great many data on the migrations and inter- 

 relations of diff^erent floras and plant groups in past geological ages, 

 besides explaining many anomalies of plant distribution of the 

 present day. On the other hand it offers no explanation of the 

 similarities of the floras of eastern Asia and North America, or of 

 the floras of islands in the Pacific which suggest trans-Pacific con- 

 nections — or at least migrations. Indeed, according to Professor 

 G. E. Du Rietz (voce), who yet believes it possible that the same 

 taxonomic entity may have arisen independently in more than one 

 area, the flora of New Zealand is contrary to the hypothesis of 

 continental drift, bearing similar relationships to both its east and 

 its west. 



The theory of polar oscillations or ' shifting of poles ', sometimes 

 called the ' pendulum theory ', also aims at explaining, on the basis 

 of incidental earlier climatic changes, some phenomena of plant 

 distribution that are far out of line with climatic zones as they now 

 exist. It is presumed that changes in climatic zones, as indicated 

 inter alia by the distributions of fossil plants, were caused by changed 

 location of the continents in relation to the sun's orientation, and 

 this theory of polar oscillations attempts to explain these phenomena 

 by assuming that periodic changes have occurred, owing to the 

 position of the geographical poles oscillating back and forth like a 

 pendulum, or at least ' wandering ' quite widely. This must not 

 be confused with the now well-known movement of the North 

 Magnetic Pole. It should be noted that continental drift and 

 (geographical) polar wandering are considered to have quite possibly 

 taken place together, in view of the plasticity implied by the former, 

 and that proponents of the latter are prone to put an earlier position 



