THE BIOLOGICAL RELATIONS OF FOSSILS, 59 



therefore corresponding differences in their "vertical range" or, 

 in other words, in the actual amount and thickness of strata 

 through which they present themselves as fossils. Some species 

 are found to range through two or even three formations, and 

 a few have an even more extended life. More commonly the 

 species which begin in the commencement of a great forma- 

 tion die out at or before its close, whilst those which are 

 introduced for the first time near the middle or end of the 

 formation may either become extinct, or may pass on into the 

 next succeeding formation. As a general rule, it is the animals 

 which have the lowest and simplest organization that have the 

 longest range in time, and the additional possession of micro- 

 scopic or minute dimensions seems also to favor longevity. 

 Thus some of the Foraminifera appear to have survived, with 

 little or no perceptible alteration, from the Silurian period to 

 the present day; whereas large and highly-organized animals, 

 though long-lived as individuals, rarely seem to live long speci- 

 fically, and have, therefore, usually a restricted vertical range. 

 Exceptions to this, however are occasionally to be found in 

 some " persistent types, " which extend through a succession of 

 geological periods with very little modification. Thus the ex- 

 isting Lampshells of the genus Lingula are little changed from 

 the Lingula which swarmed in the Lower Silurian seas; and 

 the existing Pearly Nautilus is the last descendant of a clan 

 nearly as ancient. On the other hand, some forms are singu- 

 larly restricted in their limits, and seem to have enjoyed a 

 comparatively brief lease of life. An example of this is to 

 be found in many of the Ammonites close allies of the Nau- 

 tilus which are often confined strictly to certain zones of strata, 

 in some cases of very insignificant thickness. 



Of the causes of extinction amongst fossil animals and 

 plants, we know little or nothing. All we can say is, that the 

 attributes which constitute a species do not seem to be intrin- 

 sically endowed with permanence, any more than the attributes 

 which constitute an individual, though the former may endure 

 whilst many successive generations of the latter have dis- 

 appeared. Each species appears to have its own life-period, 

 its commencement, its culmination, and its gradual decay ; and the 

 life-periods of different species may be of very different 

 duration. 



From what has been said above, it may be gathered that 

 our existing species of animals and plants are, for the most 

 part, quite of modern origin, using the term " modern " in its 



