166 



of: the Australasian and European rocks of carboniferous 

 age, for, out of a known list of about 311 Australasian species, 

 we have the high authority of de Koninck for stating that fully 

 23 per cent, are specifically identical with those of the car- 

 boniferous marine beds of Europe. In this instance the 

 results of migration from one centre does not present greater 

 difficulties than could be disposed of by the ordinary 

 interpretation of the theory of homotaxis. 



But what shall we say of the plants ? Here there is an 

 insuperable difficulty. The common interpretation of homo- 

 taxis docs not help us much, for there is not the faintest 

 correspondence between the typical plants of the same age 

 in opposite hemispheres. Indeed, it can easily be seen by 

 reference to accompanying lists of plants that there is a 

 greater characteristic relationship between the Mesozoic 

 plants of Australasia with the Carboniferous of Europe than 

 there is between the known Carboniferous plants of the 

 respective regions. 



How can we explain this anomaly ? Eor my own. part I do 

 not see what other explanation can be given than that already 

 suggested—viz., independent and widely separate centres of 

 origin, producing by slow radiating diffusion subsequently 

 into far distant regions those seemingly inexplicable compli- 

 cations where characteristic types of two separate ages 

 appear to commingle. 



This interpretation would help to explain the utter lack of 

 homotaxial parallelism between the marine and terrestrial 

 organisms of Australasia and the corresponding organisms 

 of Europe. 



The fact that there is a closer parallelism between the 

 marine remains of the two widely separated regions is pro- 

 bably due to the greater facilities for more rapid diffusion of 

 types among the marine, inhabitants of a continuous sea as 

 compared with the slower diffusion of terrestrial organisms, 

 barred as it must often have been by wide tracts of sea and 

 other physical obstructions. This conclusion is borne out by 

 the illustrious Darwin, who states (pp. 229-300, " Origin of 

 Species"): — " The process of diffusion would often be very 

 slow, depending on climatal and geographical changes, on 

 strange accidents, and on the gradual acclimatisation of new 

 species to the various climates through which thev might 

 have to pass, but in the course of time the dominant forms 

 would generally succeed in spreading, and would ultimatelv 

 prevail. The diffusion would, it is probable, be slower with 

 the terrestrial inhabitants of distinct continents than with the 

 marine inhabitants of the continuous sea. We micht there- 

 fore expect to find, as we do find, a less strict degree of 



