28 INTRODUCTION. 



And this in turn was succeeded by a form with a difference of only 



(X 0" 



That is to say, that if we compare the forms of Bryozoa that lived 

 in one particular zone, the species are discontinuous ; but if we 

 compare the forms of Bryozoa that lived in successive zones, the 

 species are continuous. Variation in this case is therefore con- 

 tinuous in time and discontinuous in space. And the latter neetj 

 not surprise us when we remember the evidence advanced by 

 Darwin in support of his argument, that the rarity of existing 

 intermediate varieties is only to be expected, for ''the very process 

 of natural selection constantly tends, as has been so often re- 

 marked, to exterminate the parent forms and the intermediate 

 links." l 



It is because the element of time necessarily enters into the idea 

 of a circulus that it supplies instructive analogy as to the nature of 

 specific groups. 



The occurrence of individuals well separated from the main 

 mass of the circulus (but still so much nearer to it than to any 

 other as to be unquestionably members of it) presents us with a 

 case of discontinuous variation ; and such discontinuous variation 

 has almost certainly taken place in the Bryozoa. 



The sudden change from Diastopora to Berenicea, as we pass from 

 the Great Oolite to the Bradford Clay (p. 20), supplies a marked 

 case of discontinuous variation. Bateson's position is that the 

 " discontinuity is not in the environment" ; whereas, as we have 

 seen, in that case it was due to a direct change of environment. 

 But such cases are exceptional. The general evidence of the fossil 

 specimens, and the great difference of opinion as to the range of 

 specific variation between those who multiply species indefinitely, 

 and those who place Silurian and recent individuals in the same 

 species tend to show that most of the forms of Cyclostomata have 

 arisen by slow, imperceptible, continuous variation. 



7. THE CLASSIFICATION OF THE CYCLOSTOMATA. 



It is the fact that continuous variation has played the leading 

 part in the evolution of the Cyclostomata, that renders the 



1 C. Darwin. The Origin of Species, 6th ed., 1882, p. 138. 



