THE PALEONTOLOGIC RECORD 21 



BIOLO.GIC PRINCIPLES OP P ALE GEOGRAPHY 



BY DE. F. H. KNOWLTON 



U. S. GEOLOGICAL SUBVEY 



/CONSIDERING the breadth and intricacy of the subject assigned 

 ^^ me, and the limited time that can be given to its consideration, 

 it has seemed best to me to restrict my remarks to two or three of the 

 obviously more important phases of the problem. 



Aside from the study of the rock-masses themselves which are 

 often difficult of interpretation relianceifor an interpretation of paleo- 

 geography must be placed in the former life found entombed, and of 

 the two biologic elements, plants undoubtedly hold a very high prob- 

 ably the highest place. 



In making use of plants in the study of paleogeography we may 

 first consider distribution. If we find two fossil floras identical or 

 similar in all essential or important details, we feel justified in re- 

 garding them for all practical geologic purposes as contemporaneous. 

 In order that we may be certain that the two floras are identical, they 

 must be composed of types that are readily identifiable, that is, forms 

 so well characterized that they may be easily and certainly recognized. 

 As examples of such floral elements mention may be made of many 

 ferns and fern allies, most cycads, conifers and peculiar, well-marked 

 or characteristic dicotyledons. Having settled the contemporaneity of 

 the floras, inquiry may next be made as to the probable manner in 

 which the separated or isolated areas were reached by these floras. 

 Here again we must carefully consider the character of the flora and 

 the means for its natural dispersal. The living flora, and for that 

 matter probably the floras from at least the beginning of the Tertiary 

 progressively to the present time, has developed in many ways means 

 for the comparatively rapid and wide-spread dissemination of their 

 reproductive parts (seeds, etc.). For example, a large percentage of 

 the members of the dominant living family of seed-plants the Com- 

 posite have developed seeds with an attachment of soft, fluffy hairs 

 which serve to float them in the air, often to great distances. In many 

 other living groups there are similar, or at least as effective, devices 

 for dissemination, but as we go back in time adaptations calculated to 

 be of aid in distribution grow less and less, and soon even seeds of any 

 kind are unknown, or known but imperfectly, and reproduction is 

 normally by means of spores, that is, reproductive bodies in which there 

 is no embryo already formed when they leave the parent plant. It is 

 obvious that plants that are reproduced by seeds, in which there is 

 both an embryo and a supply of food for use during germination, must 

 possess a decided advantage over those reproduced by means of spores. 



