^^^^l DISCUSSION OP DIAGRAMS. 443 



In the second place, we can consider each type of vegetation by and 

 in itself, with a view to determining the geological age in which it first 

 made its appearance, the general nature of its progress through time, 

 and the period of its maximum actual development as an element of the 

 vegetation. Such a presentation, however, when based on the number 

 of species actually found at each horizon, exhibits very great fluctua- 

 tions, due to the irregularities in the record. These irregularities de- 

 pend chiefly upon conditions quite independent of the real presence or 

 abundance of the plants in any formation. These conditions are many, 

 but the principal ones may be embraced under three heads : 1. The 

 plants must have existed at the period in question. This is the legiti- 

 mate assumption and alone gives value to the diagram. 2. The condi- 

 tions for their preservation and then for their subsequent exposure must 

 have occurred. Any one can see how exceedingly irregular must be 

 these delicate conditions at different ages of the world. 3. The locali- 

 ties in which they are embedded must have been discovered and worked 

 by the paleontologist. This is the great contingency which stands in 

 the way of our acquaintance with any flora, but although doubtless 

 more potent than the one last named, it possesses the merit of possible 

 removal through the industry of man. 



With all these detractions from its value this form of illustrating the 

 geological record is nevertheless presented in Diagram No. II. 



In the third place, we may, by a legitimate exercise of the rational 

 method of science, construct a scheme of illustration, based indeed upon 

 these facts as indispensable landmarks, yet recognizing the law of uni- 

 formity in natural processes that constitutes the primary postulate of 

 science itself, which shall, to a large extent, eliminate the error of the 

 defective record and present a rational and highly probable view of the 

 true development. By a second act of ratiocination the probable period 

 of first appearance of each type of vegetation may be deduced from the 

 fact as to the earliest point at which it has actually been discovered, 

 and thus an approach far nearer, at least, to the true history of plants 

 than is possible by the last-named method may be made. Diagram No. 

 Ill presents the subject from this third point of view. 



Discussion of Diagram No. I. — In this diagram the Cryptogams are 

 represented in buff tints and the Phanerogams in purple, with deeper 

 shades for the successively higher types of each series. The diagram is 

 based upon the assumption of the proportionate representation of types 

 in the known floras of each age. Collectors of fossil plants never select. 

 They take everything they find and make no attempt to find particular 

 forms. If, therefore, the chances of preservation of different kinds of 

 plants were equal the chances of finding any particular kind would de- 

 pend upon its actual degree of abundance in the given flora. Con- 

 versely, the degree to which any type of plants is represented in the 

 collections made would be a fair measure of such abundance or of the 

 relative prominence of the type in the flora of the given epoch. How- 



