100 INTRODUCTION. 



ance of groups of morphological types in corresponding geological 

 periods in widely separated areas of the earth's surface is probably 

 fallacious. Such groups must have appeared first in one area, and 

 their extension therefrom must have been the result of subsequent 

 migration. The only other explanation of this phenomenon would 

 be that the same morphological types had been simultaneously pro- 

 duced at several widely remote points ; but this hypothesis is appar- 

 ently irreconcilable with any theory of evolution. 



6. Extinction of Morphological Types. — While new species have 

 been constantly appearing throughout geological time, old species 

 have as constantly been undergoing extinction. In some cases, 

 extinction seems to have taken place with extraordinary abruptness, 

 as seen, for example, in the sudden disappearance of the Rudistce. at 

 the close of the Cretaceous period. In other cases, extinction has 

 been a gradual process. In either case, we are to a large extent 

 ignorant of the causes of extinction, and of the laws under which 

 the process is carried on. As a general rule, it may be taken for 

 granted that the sudden disappearance of a whole group of morpholo- 

 gical types is more apparent than real. When we have a sufficiently 

 complete series of deposits, it is usual to find that a group has 

 begun to dwindle down long before it finally disappears from the 

 scene. Thus, the Graptolites, the Trilobites, and the Orthocera- 

 tites exhibit a progressive diminution as regards the number of 

 specific types before we reach the point at which extinction takes 

 place. Why these, and other similar groups, should show such an 

 extraordinary power of rapid extension and of the development of 

 new specific or generic types when first introduced upon the earth, 

 and should thereafter progressively decay and ultimately become 

 extinct, is a problem for which the solution has yet to be found. In 

 any case, it is to be remembered that in many cases " extinction " 

 implies nothing more than continued existence under a new form. 

 That is to say, a species often becomes apparently extinct by becom- 

 ing gradually modified into a new species, in which case the parent- 

 form actually disappears, but the modified form represents it in later 

 deposits. 



7. Evolution of Morphological Types from pre-existing Forms. — 

 Palaeontology has furnished a mass of evidence in favour of the view 

 that the introduction of new species in past time has been by evo- 

 lution from pre-existing forms. Taken as a whole, therefore, the 

 evidence of Palaeontology points to the operation of some general 

 law of evolution, whereby the later forms of life have been derived 

 from the older ones. The principal palaeontological facts which sup- 

 port the general theory of the evolution of organic types from pre- 

 existing forms may be briefly glanced at here under the following 

 heads : — 



