276 Practical Plant Biology. 



introduction. The fact that the islands have been of comparatively 

 recent formation shows that the transformations of species occur 

 within limits of time which make these changes of prime importance 

 in the evolution of organisms in the history of life upon the earth. 



(3) Evidence from the Geological Record. 



Decisive evidence on the mutability species would be afforded if 

 we possessed complete collections of the flora or fauna of some 

 district extending back over a sufficiently long period. In such 

 collections, herbaria of dried plants for example, if species were 

 immutable we would find the plants between the earliest pages 

 exactly corresponding in their specific characters with the most 

 recently collected specimens, while if they were mutable the recent 

 specimens would be found to diverge from those between the 

 earlier sheets in proportion to the time elapsing between the two 

 periods of collection and to the mutability of the species con- 

 sidered. 



While scientific collections do not extend over nearly great 

 enough periods to show great transformations, and are not by any 

 means sufficiently complete to establish small changes, the sedi- 

 mentary rocks of the earth's surface do form such chronological 

 collections which extend into sufficiently remote periods to record 

 the great transformations. Unfortunately the record is so incom- 

 plete that the minor and more gradual changes are but seldom 

 registered in what is preserved. The strata of these rocks the 

 pages of the majestic volumes of the Book of Time were laid 

 down on the margins of past continents and in great inland seas. 

 They contain the remains of past floras and faunas, which lived and 

 died on the surface of sediments accumulated before them, or were 

 carried short distances to be laid between the leaves of nature's 

 record. Here, in fortunate instances, they were preserved by the 

 infiltration of mineral solutions and now form fossils giving us some 

 indication of the remote ancestors of our present living beings. 

 The study of these fossils clearly establishes that the further we 

 turn back the pages of the geological record the more widely does 

 the flora preserved there differ from that which is now around us. 

 In recent rocks we find the remains of existing orders and classes. 

 More seldom have existing genera and species been found. In 

 earlier sediments no records of flowering plants are seen, while ferns 

 and even ferns producing seeds are plentifully represented. In the 

 earliest rocks of all not only flowering plants but even ferns leave 

 no record, and the only specimens found are such as belong to the 

 more simple and primitive groups. A parallel record of animal 

 remains, more complete seeing that animal structures lend them- 



