98 INTRODUCTION. 



sation in the introduction of new species is probably mainly due to 

 the shortness of the period during which accurate observations have 

 been carried on. Moreover, though our present species have existed 

 for a time which, relatively to man, is very long, this time, estimated 

 geologically, may be, and probably is, very short. It is, further, ex- 

 ceedingly probable that the introduction of new species would, under 

 any circumstances, be imperceptible to a single observer, or even to 

 many successive generations of observers, since the changes by which 

 species are evolved from pre-existing species are probably so slowly 

 produced as to be imperceptible except when fully completed. 



3. Abrupt Appearance of New Species. — In a large number of in- 

 stances new morphological types appear to have come into existence 

 abruptly, no closely allied types being known to have preceded them. 

 This is necessarily the case with the animals of the most ancient of 

 the fossiliferous formations (viz., the Lower Cambrian) ; but a similar 

 phenomenon is observable in hosts of other instances, where we 

 might reasonably expect to find that the new types were preceded by 

 older relatives. It is obvious, however, that this apparently abrupt 

 appearance of a new morphological type — as, for example, the sudden 

 appearance of the great family of the Rudistce in the Cretaceous 

 period — arises from an imperfection in our knowledge. On any 

 theory of evolution, each morphological type must have come into 

 existence, coincidently, both in space and time, with a pre-existing 

 allied morphological type (Wallace). When, therefore, we find new 

 morphological types suddenly appearing on a given geological horizon, 

 in an area where no allied forms have been found in older de- 

 posits, we must come to one or other of two conclusions. Either 

 the apparent absence of allied types in older strata is due to the fact 

 that these strata have not been sufficiently investigated, or the appar- 

 ently sudden introduction of the new forms is due to the fact that 

 the case is not one of their first coming into existence at all, but 

 simply one of their first appearance in the area under observation. 

 In many instances where new organic types suddenly appear in an 

 area, the older deposits in which we might expect to find the remains 

 of the predecessors of these, are missing altogether in the same area. 

 Thus, we cannot expect to find the immediate predecessors of the 

 numerous new life-forms which in Europe usher in the commence 

 ment of the Secondary period, till we find the deposits which were 

 laid down in the interval between the close of the Permian and the 

 beginning of the Triassic period. In many other instances, however, 

 where the series of the stratified deposits may be moderately com- 

 plete, the apparent abruptness of appearance of a given morpholo- 

 gical type is really due to the fact that the series has been imper- 

 fectly examined ; and further investigation would either show that 

 the type in question really began to exist at an earlier period, or 



