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A comparison of the various substances of which fossils 
are composed leads to the conclusion that very little of the 
original matter of the hard parts is preserved, and that very 
generally the fossil is a mere cast of the original, filled with 
whatever mineral happened to be in solution in the stratum 
in which it is imbedded. In some cases the cast exhibits 
the minute structure of the original, as in the case of the 
Yorkshire hazel nuts in the Oxford Museum, in which the 
kernels have been converted atom by atom into calcite 
without the cellular arrangement of the oi'iginal being dis- 
turbed, and without the shell being altered in any degree. 
The fact that our knowledge of animal life in past time 
depends principally on mere casts of the hard parts which 
happened to be imbedded in the strata demonstrates the 
truth of Mr. Darwin’s view that the geological record is 
very imperfect. 
Professor W. C. Williamson, F.R.S., exhibited some 
specimens of Stigmaria, and indicated their bearing upon 
views advanced even by the most recent writers on the sub- 
ject. He demonstrated that the centre of the axis was 
occupied by a pith of delicate parenchyma, wholly devoid of 
the vessels described and figured by Goeppert, and which 
certainly never belonged to the part of ‘the plant in which 
he figured them. The lenticular spaces long known to exist 
in the lignous zone surrounding the medulla, Professor Wil 
liamson showed to be true medullary rays, occupied by 
mural cellular tissue prolonged directly from the medullary 
parenchyma. Besides these, smaller or secondary medullary 
rays separate many of the individual laminae of the vascular 
tissue. He then pointed out the truo source of the vascu- 
