PALEONTOLOGY AS A DISCIPLINE. 51 



palaeontology consists in the uncertainties of geological cor- 

 relation, by which the relative age of formations in widely 

 separated areas and different continents is to be determined. 

 It may and often does make a vital difference in the construc- 

 tion of a phylogeny, whether a given set of rocks in North 

 America is older or younger than one in Europe, with which it 

 is correlated. The principles according to which such corre- 

 lation is to be made are still somewhat indeterminate, and not 

 a few geologists maintain that the problem is an insoluble one. 

 On the other hand, it is essential to the palaeontologist that it 

 should be solved, and already a very encouraging beginning 

 has been made. 



(4) In the fourth place the apparent order of succession of 

 organisms in the stratified rocks must not be too implicitly 

 and uncritically accepted. Animals and plants diffuse them- 

 selves as widely as possible until stopped by some impassable 

 barrier. During the long ages of the world's history these 

 migrations have ever been in progress, and they greatly confuse 

 the record when we attempt to read it in terms of evolutionary 

 descent. A species in a newer formation, which appears to be 

 derived from one in an older horizon of the same region, may, 

 as a matter of fact, have had an entirely different ancestry and 

 have migrated half around the globe to the place where it 

 occurs. To make these distinctions theoretically is easy, to 

 apply them very difficult. 



(5) Lastly should be mentioned a practical drawback to the 

 palaeontological method, namely, its costliness. The naturalist 

 may find much to do in other departments at small expense, 

 which will be a source of infinite pleasure to himself and of 

 great value to science. Every field and wood, every pond and 

 stream, and above all the sea, offer boundless stores of ma- 

 terial. Even the side of palaeontology which bears upon 

 stratigraphy and historical geology may be taken up to 

 great advantage by the private worker who happens to live 

 in a favorable locality. With palaeontology as a branch of 

 morphology, however, the case is unhappily very different. 

 Here great collections brought together without much regard 

 to cost, skilled workers to prepare the specimens, and great 



