INTRODUCTION. 27 



4. Continuous Variation. Comparisons between the formulae of 

 allied species belonging to different geological periods show that 

 the differences between them are often very small. The species 

 are often so much alike that it is impossible to indicate the 

 distinctions by general diagnoses, for there appears to have been 

 a continuous variation from one species or specific group to the 

 next. We cannot, however, refer to such continuous variation 

 without considering Mr. Bateson's l recent discussion of the nature 

 of variation, and his conclusion of the greater importance of discon- 

 tinuous than of continuous variation in the origin of new species. 

 ]\Ir. Bateson has shown (e.g., op. cit. p. 41) that the individuals 

 included in a species are not distributed uniformly among 

 the different variations included with the limits of that species. 

 The great majority of the members of the species agree with one 

 or two types, and comparatively few members are scattered near 

 the bounds of the limit of variation, or far from the main centres 

 of organic stability. This is exactly the idea of a circulus. Each 

 circulus is a "centre of organic stability," and the variation 

 around it is of continuous and almost imperceptible gradations. 

 Bateson takes as his first axiom 2 that " The forms of living things 

 are various, and, on the whole, are discontinuous or specific." 

 He concludes 3 "that the discontinuity of species results from 

 the discontinuity of variation." Elsewhere he tells us that 

 ' ' such discontinuity is not in the environment : may it not, then, 

 be in the living thing itself?" 4 



The study of the Cyclostomata does not appear to me to support 

 these views. Bateson's axiom is doubtless true for living Bryozoa, 

 and was doubtless true for the Bryozoa that lived at any one 

 previous epoch. But it is not true if we compare the Bryozoa of 

 successive faunas instead of different individuals of the same fauna. 

 The nearest contemporary ally of Stomatopora dichotoma (Lamx.) 

 differs from it by an amount expressed by formula as 



1201 



The nearest ally of that species, found in the succeeding geo- 

 logical period, differs from it only by 



0' 0' 



1 "W. Bateson. Materials for the Study of Variation, treated with special 

 regard to Discontinuity in the Origin of Species. 1894. 

 a Op. cit. p. 3. 3 Op. cit. p. 568. * Op. cit. p. 17. 



