164 



With such important additions to our knowledge of the 

 flora of the Mesozoic rocks of Tasmania, it becomes a ques- 

 tion of much interest to enquire how far they shed light 

 upon disputed questions regarding the exact age of the beds 

 in which they are found, and therefore of the age of the 

 upper coal measures of Tasmania a.nd Australia generally. 



The comparative tables given showing the wide range of 

 distribution of most of our typical Mesozoic genera indicate 

 that parallelism with the subdivision of distant regions is 

 not such a simple question as some authorities seem to think. 

 It is beset with many fundamental difficulties, among which 

 I cannot too strongly urge the consideration of centres of 

 origin. 



Have we any just reason for supposing that there were 

 many centres of origin from whence sprung independently 

 similar genera, and if so, have we also good reasons for 

 supposing that from these several centres identical forms 

 sprang into existence in opposite hemispheres at the same 

 point of time, or even within the same geological period ? 



Those who agree with Darwin in referring the origin of 

 higher forms of life to one pair (an hypothesis which the 

 author inclines to) would at once reject such a supposition 

 as being opposed to the best evidence. But even should it 

 be admitted that the higher species and genera sprang from 

 one particular pair, have we at this moment sufficient 

 evidence to show, with anything like certainty, at what point 

 of time and in what spot of the earth's surface each particu- 

 lar genus came into existence ? 



When we consider how small a portion of the earth's sur- 

 face has yet been examined with sufficient care, and how 

 large is the space comparatively unexplored, he must be bold 

 indeed who would allege that those centres (principally 

 European) where certain species or genera are first known to 

 appear are also the centres of their original appearance as 

 regards the face of the whole globe. 



Further, the question of distribution over wide areas requires 

 special consideration. It is well enough known that species 

 of the lower forms of plant life, such as ferns and mosses 

 amid the vicissities of change of situation, have spread over 

 both hemispheres without such a departure from the central 

 type as would constitute a distinct species ; but is this true 

 as regards higher forms, such as cycads and conifers ? And 

 even where this question can be answered in the affirmative 

 can we be certain that the distribution of a given species 

 from, say, one hemisphere to its antipodes would be effected 

 within a single geological period, not to mention the diffi- 

 culties of transfer — natural and physical — which would 



