176 NATURAL SCIENCE. March, 



adaptive meaning at all, either in its generic extent or in the specific 

 differences. Nor, as already shown, is there evidence that any advan- 

 tage is conferred on the fish by the perforation of the gill-septum. 



The utmost that can be admitted then as adaptation in these fish 

 is the shape of the body and breadth of the fins behind. The linear 

 extent of the fins is as great and greater in many other genera. The 

 accessory flaps beneath the tail, and the extent of the pelvic fins, 

 cannot be regarded on present evidence as of adaptive meaning ; still 

 less can the specific differences. 



The relations of these three species, on the view that their 

 differences are not adaptive, illustrate certain principles which have 

 been elaborated by Romanes and Eimer. They exemplify the class 

 of facts which Romanes intended to explain by his theory of physio- 

 logical selection, which, reduced to its simplest terms, states that 

 species whose geographical ranges overlap or to a large extent coin- 

 cide, could not become distinct if they were constantly interbreeding. 

 Thus Z. punctatus and Z. norvegicus occur together in Norway and 

 Britain, but do not interbreed, and Z. nnimaculatus, though alone in 

 the Mediterranean, lives with the others from Denmark southward. 

 Whether these species are sterile inter se, whether fertilisation of one 

 by the other is physiologically impossible, whether their reproductive 

 seasons are such that opportunities for intercrossing do not occur, or 

 whether the intercrossing is prevented by the mating instinct of the 

 individuals, is not at present known, and does not very much matter. 

 The main point is that so long as intercrossing took place, variations 

 that occurred in one group of individuals would sooner or later become 

 the common property of all the members of the parent form ; and that 

 when the three groups were kept apart, a variation that arose in one 

 would be confined to that one. We may take it as settled by observa- 

 tion that when groups of a species are isolated they will diverge by 

 variation. It may be asked, why should two groups of individuals, 

 having originally the same habits and spread over the same area, enter 

 upon divergent lines of modification, simply because they are 

 separated ? We do not know, but we can safely say that they will 

 do so. We have first to ascertain what does take place before we 

 can find out why it takes place; and when we find that the differences 

 between species are not differences of adaptation to different modes of 

 life, we have simply to study these differences and their history as 

 structural features. It is clear that in artificial breeding, separation 

 of varieties is one of the most important conditions, and there is 

 abundant evidence of the same separation in nature. The term 

 selection often seems to imply this separation, but inasmuch as 

 selection always means principally the preservation of certain 

 individuals in a given variety or race, by human choice in domestica- 

 tion, by the struggle for existence in nature, and therefore always 

 implies a teleological view of structure, therefore it is better to use a 

 distinct term, and the best seems to be "isolation." We may 



