582 THE TERTIARY FLORA OF AUSTRALIA, 



■entirely depended on. Thousands of species have been created 

 and their affinities declared on the evidence afforded by leaves 

 alone, and often with an imperfect knowledge of the existing 

 flora of the region and a preconceived desire to prove a certain 

 theory. 



The character of leaf-venation would prove an unerring guide 

 to the determination of fossil plants if those characters were 

 arranged through the vegetable kingdom on any orderly plan. 

 If the leaves of all plants of a particular natural order possessed 

 ■a, common general character, and if those characters became more 

 more defined and specialised as the genera were dealt with — if, 

 finalty, the specific differences were less than the generic differences, 

 and still less than those of the ordinal diffei'ences, leaves would 

 be as important a guide in the work of botanical classification as 

 the parts of the flower, and the value of any particular fossil 

 leaves would only depend upon the perfection of their preservation, 

 as the affinities of those in which the substance and venation 

 remain intact could then be absolutely determined. 



One does not need to go far through the vegetable kingdom to 

 find that leaves are not constructed on this methodical system, and 

 almost immediately it will be recognised that not only do the 

 leaves of different species of the same genus often differ very 

 greatly from one another, but that their venation seems to be laid 

 out on a totally different principle. On the other hand, leaves 

 constructed on the same pattern and often undistinguishable are 

 to be found in widely different natural orders. 



The wonderful amount of variation in leaf-venation is a subject 

 to which due weight is not alwa} r s given, and those who are not 

 botanists, but yet scientific men, can scarcely be blamed if they 

 have been led to suppose that the structure of leaves has more or 

 less followed in its development the general scheme of the higher 

 divisions of the vegetable kingdom. The use of such specific 

 names as " quercifolia," " ilicifolia," " acerifolia," " cinnamomi- 

 folia," &c, tend, at least, to give rise to the impression that the 

 leaves of the various species of oak, holly, maple, cinnamon, &c, 



