28 Missing Chapters of Geological History. 



be not real species, and indeed whether true species exist or 

 not, are sufficient, and to be depended on. What is sought is a 

 modification of external form that shall indicate a change in 

 conditions of existence. A close resemblance of structure is 

 proof of absence of change in conditions of existence, while a 

 marked change of structure affords equal proof of such change. 



In examining the Secondary rocks, we find alternately small 

 and larger varieties in the proportion of species that pass from 

 one to another. In one place we find fifty per cent, of the 

 species common to the lower and overlying rocks ; in others 

 there are passages where most of the Ammonites, or Belem- 

 nites, or Terebratulas, are continuous, but other shells change ; 

 or where the majority of species continue, but some of marked 

 importance are altered. All facts of this kind are lessons, and 

 they are the best and truest lessons afforded by modern 

 geology. The more complete the series the more carefully 

 can they be studied. 



But when we have learnt the meaning of a small part of 

 a small formation developed in the ordinary way in a single 

 district, and observe the degree of change thus produced ; 

 when we find that of each hundred species, or marked varieties, 

 of animals or plants contained in the middle of such a deposit, 

 only about fifty come up from the formation of a similar kind 

 immediately below, although no stratigraphical interruption 

 can be traced, while only twenty pass into the rocks above, 

 with equally little evidence of disturbance ; we shall see that 

 time alone is capable of inducing a great change, or, at least, 

 that time, combined with such changes as leave no mark, 

 has done this thing. We then see the value of time, and 

 are able to comprehend the next step, namely, that when there 

 is marked change in mineral condition in two rocks, one 

 overlying the other, the difference in species is much greater. 

 This perhaps may seem less surprising, for there may have 

 been migration, and the old species driven away for a time may 

 return. They do occasionally but rarely so return, but never 

 in large numbers. Time again has acted, and time, as well as 

 circumstances, change species. 



When, finally, we come to a total difference, a true paleeon- 

 tological break, accompanied it may be by a stratigraphical 

 break, so that not more than from two or three up to fifteen 

 or twenty out of every hundred species pass from a rock below 

 to an unconformable stratum above, we feel that the lapse of 

 time now required must have been proportionably great. 

 When the lower rock has evidently been not only formed but 

 dried and hardened, and sunk down to the depth at which 

 metamorphic change is active ; when it has there been con- 

 verted into a different mechanical and mineral combination, 



