HISTORY OF THE VEGETABLE KINGDOM. 669 
remains being generally in the form of logs, stumps, fruits and leaves, which are 
most generally embedded in the limestones and sandstones of that age. They 
were doubtless upland productions, and those that are preserved as fossils were 
probably driftwood that had been carried down by flocds and buried in the 
alluvial deposits of that time. Though these were without doubt conifers, they 
bore little resemblance to modern genera and seem to have been very generalized 
types. They appear to have borne a considerable resemblance to the Auricaria 
or Norfolk pines of the present day. 
Such is a very general outline of the flora of the coal period. A few facts 
in connection with this flora are worthy of more extended consideration. The 
first fact that arrests attention is the vast amount of vegetable matter that must 
have been produced during thisage. It has been estimated that it requires not 
less than eight to twelve feet of vegetation to make one foot of coal, and that a 
vegetable production of two tons per acre per annum would, when the lighter 
gases—hydrogen and oxygen—are eliminated, as is always the case in forming 
coal, produce only one-fourth of an inch of coal in a century. At this rate it 
would require about 5,c00 years to form one foot of coal. Now, in some 
localities there are beds of 40 to 50 feet of almost pure coal, while in places the 
aggregate of the different beds reaches 100 and even 150 feet of solid coal. 
This would indicate an enormous lapse of time, or a luxuriance of vegetation of 
which we can form no conception. Probably both these causes operated to pro- 
duce these results. 
The next fact to be noticed is the highly differentiated character of the flora 
of the carboniferous age. We have here in contemporaneous existence the Thal- 
logens of earlier ages, the Acrogens for which this age is specially noted, and the 
Exogens that were more conspicuous in later times. 
Though these all flourished contemporaneously, it has been assumed by some 
that the more highly developed forms were derived from the more simple by a 
process of development. But while there has been a general advance in structure 
of vegetable organisms, there is a lack of evidence of related successional forms 
that would seem to be essential to sustain such a theory. According to the 
evolution theory, the various types of plants should have appeared in the order 
of their complexity of structure: First, the Thallogens, next the Acrogens, 
next the Endogens, followed still later by the Exogens. And not only should 
they have appeared in this order of succession, but there should have been 
gradational forms to show the successive steps by which the lower rose to the 
higher forms. But such intermediate forms are not forthcoming. 
The higher types appear suddenly and without any appearance of progenitors 
of any earlier types. The assumption that such development must have taken 
place during the long lapse of time that is supposed to have intervened between 
the different eras and of which we have no record left in the rocks, is certainly 
not consistent with the rules of evidence required by scientific reasoning. It is 
assuming that the theory must be true simply because we do not know it to be 
