96 EVOLUTION OF LIVING ORGANISMS. 



both would seem to be doomed to destruction in the end 

 by an unfavourable environment when this earth becomes 

 too hot or too cold to support life. In the meantime, it 

 is the great merit of the Darwinian principles of evolu- 

 tion that they account for the failures as well as for 

 the successes. The elimination of the unfit, leaving the 

 better adapted in possession, is a necessary part of the 

 process. 



The living organisms of to-day show us the types 

 which have succeeded ; for the failures we must appeal 

 to the record of the past as revealed by a study of fossil 

 remains. This record, in spite of its incompleteness, 

 has much to teach. Every theory of evolution must be 

 tested by the results of palaeontology ; no conclusion can 

 be accepted which is inconsistent with them. 



In the first place, the conviction derived from a study 

 of living forms is confirmed, that evolution does not pro- 

 ceed along continuous straight lines, but, on the contrary, 

 along a multitude of diverging branches. Just as indi- 

 viduals are found to vary in all directions compatible 

 with their structure and composition, so groups become 

 differentiated in various directions, each adapted to a 

 particular mode of life. Having reached a certain favour- 

 able combination of characters, they start on this new 

 plane of structure to diverge, according to the principle 

 of adaptive radiation, as it has been called by Osborn, 

 many instances of which are found in the history of the 

 land vertebrates. 



Derived from some fish-like aquatic ancestor in 

 Devonian or pre-Devouiaii times, the land vertebrates 

 appear in Carboniferous strata as clumsily-built Am- 

 phibia with four walking limbs. Like their modern 

 representatives, they spent their early life in water, 

 breathing by means of gills, and made use of lungs for 

 respiration in adult life on land. These primitive 

 amphibia soon diverged in various directions. Some 

 acquired a large size and formidable dentition, like the 



