xiv. SUCCESSION OF LIFE-FORMS. 405 



Examples not less instructive, and all tending to the same con- 

 clusion, may be taken almost ad libitum from all the races of marine 

 animals. In hundreds of instances we can trace backward in time 

 the characteristic elements of generic structure to the earliest 

 known type : in a small number of cases these lines of representative 

 life, these probable genealogies, extend through all or nearly all 

 the vast period which is known to us with certainty under the 

 titles of palaeozoic, mesozoic, and cainozoic life ; a period which can 

 only be expressed by the inconceivable symbols of a million, ten, 

 nay a hundred millions of years. Yet during all that immensity 

 of time, through all the physical changes which have happened 

 to inorganic nature, lingula and rhynchonella have existed with 

 little real difference, as if to shew the narrow limits within which 

 modification by descent is restricted. 



Take other cases and very many can be taken when the group 

 to be examined is not living and has not been found among the 

 earliest types, in these also the successive forms appear to have 

 been subject to the same law of limited, though perhaps more 

 considerable, changes ; and the earliest types deviate in no essential 

 points from the general plan of the family. 



In all cases, then, we find for each race what appears an im- 

 passable point of its history its first appearance among the relics 

 of life, with all its essential characters. The world of life consists, 

 or seems to consist, of a great number of families occupying a large 

 range of time, and in this range admitting of many variations, 

 mostly of slight (as terebratula), but sometimes of considerable 

 extent (as trigonia) ; distinct from one another in all the course of 

 their history ; separate in their origin ; often beginning their course 

 at different epochs of time. How they came into existence we know 

 not, nor can with much hope of success conjecture. For nothing 

 known of the present inhabitants of the sea or land furnishes 

 analogies of much cogency in this dark quest, though in regard 

 to modification of form in descent there is plenty of experimental 

 evidence in the living creation to help the palaeontologist ; as, on 

 the other hand, the long history which he offers of what nature 

 has done, what has been obtained by differentiation through a 

 hundred millions of years, must ever tend to keep within right 

 limits the attractive speculations of the biologist. 



In a treatise like the present, which aims to gather from a limited 



