i8 9 5. ORIGIN OF SPECIES AMONG FLAT-FISHES. 239 



Reptilia, and that the other divisions of that class are not separated 

 from one another by adaptive characters only. The facts I have 

 given do in my opinion tend to confirm most strongly the conclusion 

 that an explanation of adaptation is not an explanation of the 

 structural relations of species, genera, or even higher divisions. I 

 hold that Romanes' view, that indifferent or non-adaptive characters 

 have been eliminated by natural selection in the differentiation of the 

 higher divisions, is not a full expression of the truth. I think, how- 

 ever, that the discontinuity of species is sufficiently explained by the 

 view of Romanes and others concerning the effects of the isolation 

 of strains or races, whether by geographical, environmental, or 

 physiological barriers to interbreeding. In my opinion, therefore, 

 discontinuity in variation is not necessary, as Bateson main- 

 tains, to explain the discontinuity of species. However gradual 

 may have been the process of modification by which two allied 

 species have been derived from a common ancestral species, the 

 existing discontinuity is a necessary consequence of the divergence 

 of the two lines of modification, provided that no intermixture 

 of the variations in the two lines has taken place. At the 

 same time, I consider that the investigation of the modes in 

 which variations actually do take place is of the highest importance, 

 and that Bateson's contribution to this investigation is of very great 

 value. The only general view, as it seems to me, which can be held 

 concerning the structural diversity of the animal kingdom is to regard 

 it as the resultant of two more or less opposing general tendencies. 

 On the one hand, there is universal evidence of a tendency to 

 definite variation, or growth in different directions, leading to mani- 

 fold variety of regular definite symmetrical forms. This tendency can 

 only be regarded as internal to the organism, as connected with the 

 tendency to growth and multiplication inherent in organic units. On 

 the other hand, there is the moulding, limiting, constructing action of 

 the external forces of the environment resulting in more or less 

 complete adaptation. Whatever be the process of adaptation, 

 whether Darwinian selection or Lamarckian modification, adaptive 

 structural combinations are mechanisms, each working with a parti- 

 cular result which is important to the feeding, living, and breeding of 

 the organism. Whatever the causes of non-adaptive variation, the 

 resulting structural features are the regular "geometrical " forms and 

 characters which the multitude of different organic forms present in 

 such marvellous diversity. No one who, like Weismann, ignores 

 everything except adaptation, or who, like Bateson, regards the study 

 of adaptation as barren and profitless, can hope to produce a consistent 

 and comprehensive theory of organic evolution. 



Plymouth. J. T. Cunningham. 



