20 THE THEORY OF EVOLUTION 



in which we meet them again for the first time, show an 

 entirely peculiar appearance, so that at the first glance 

 no one would think of any connection with the fauna 

 of ( a/ If, however, the groups of formation ' a ' show 

 a distinct tendency to vary in a definite direction, and 

 if from a comparison of the fauna of ' c ' with that of ' a ' 

 it is seen that the heterogeneousness consists in a great 

 but apparently interposed increase of just those varia- 

 tional tendencies evinced in ' a/ then it may be assumed 

 with great probability that the organisms in ' c ' are the 

 modified offspring of those of ' a/ The intermediate 

 links lie buried in ' b/ and this formation is possibly 

 now and has been for a long period covered with water 

 and therefore inaccessible to us. The same process can 

 also be repeated for ' c/ One of the fauna of. ' c 3 approxi- 

 mating thereto, but greatly modified, may for instance 

 only be found again in an obviously much younger 

 formation, say in ' f / and this may be in North America 

 while ' c ' may be in Europe. (The strata ( d ' and ' e ' lie 

 perhaps under water or have not been investigated.) If 

 we accept a connection between ' f/ ' c/ and ' a/ then we 

 have obviously only important outposts as it were in 

 the march of evolution of a particular group, and 

 perhaps also a general indication how the outpost ' f ' 

 can have been derived from ' c ' and ' a/ but without 

 any precise knowledge of the process involved. 



Only in a few cases : as, for example, the same sea 

 in which the formation ' a ' was deposited, may, in a 

 short time, return to its old position (sea oscillations), 

 and consequently the same organisms also return, so 



