58 PRINCIPLES OF PALEONTOLOGY. 



ment of a great formation 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 

 organisation that have the longest range in time, and the 

 additional possession of microscopic or minute dimensions 

 seems also to favour longevity. Thus some of the Forami- 

 nifera appear to have survived, with little or no perceptible 

 alteration, from the Silurian period to the present day ; whereas 

 large and highly-organised animals, though long-lived as indi- 

 viduals, rarely seem to live long specifically, and have, there- 

 fore, 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 existing 

 Lampshells of the genus Lingula are little changed from the 

 Lingulce 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 attri- 

 butes which constitute an individual, though the former may 

 endure whilst many successive generations of the latter have 

 disappeared. 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. 



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

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

 part, quite of modem origin, using the term '-modern ' ; in its 

 geological acceptation. Measured by human standards, the 

 majority of existing animals (which are capable of being 

 preserved as fossils) are known to have a high antiquity ; 

 and some of them can boast of a pedigree which even the 

 geologist may regard with respect. Not a few of our shell- 

 fish are known to have commenced their existence at some 

 point of the Tertiary period; one Lampshell (Tcrcbratulina 



