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Part III. — Eighteenth An nurd Report 



The question then appears — How have these alterations in the relative 

 proportions of the North Sea plaice arisen? Why should the specific 

 variability of the organism not give rise to the same relative proportions 

 over the whole area 1 Two formal answers are ready to hand, both of 

 which take for granted that certain differences must exist in the 

 surrounding conditions. On the one hand, natural selectionists would 

 have it that these relative proportions of structures and organs are 

 "adaptive" — i.e. v that each combination has been selected by its environ- 

 ment as the best fitted for the conditions around. On the other hand, 

 we might say that the differences in the environment have directly caused 

 the differences in the proportions. The various combinations of the 

 proportions are all adapted, it might be said, to the environment, and 

 natural selectionists can suggest no reason why the one combination should 

 appear in one place and another combination in another. 



Many arguments might be brought forward in favour of both of these 

 formal explanations, but as they are based only on circumstantial 

 evidence* any discussion is premature. It is profitable, however, to 

 consider whether these alterations in the proportions of the North Sea 

 plaice may not have arisen under exactly similar external conditions. 

 We have accounted for the changes in the Baltic plaice. The environ- 

 ment is different there from that of the North Sea. But in the plaice of 

 the latter region we have not the same evidence for differences in the 

 environment. In the former case the average numbers of vertebrae and 

 fin-rays were greatly reduced, and these were connected with a decrease 

 of salinity and an increase of temperature. In the latter case the average 

 numbers of vertebras and fin-rays remain the same over the whole region, 

 and we might thence conclude that the average salinity and temperature 

 remained also the same. We have no reason, further, to conclude that 

 the remaining external conditions — pressure, physical and chemical con- 

 stitution of the water, food supply — are in any way different in the 

 regions referred to. They may be, but we lack evidence. 



If the conditions are the same, how may we account for the alteration 

 in the proportions of the northerly and southerly North Sea plaice 1 A 

 possible explanation may be found in the action of man. The southerly 

 regions of the North Sea have been more rigorously fished for centuries 

 than the northerly regions. Various effects of this rigorous fishing have 

 been already mentioned. The number of large specimens decreases, the 

 average size of the mature specimens is therefore lowered, and possibly 

 also the average-size when they become ripe for the first time. The effect 

 of this would probably be that the specimens which survived would be of 

 a younger type than formerly. Now, it has been shown that in certain 

 respects the plaice of the southerly North Sea might be taken for younger 

 specimens of those of the northerly North Sea. The differences which 

 contradict this are shown in the smaller relative body-height and correlated 

 dimensions of the skull in the plaice of the southerly North Sea. It has been 

 demonstrated also that these dimensions are correlated with the develop- 

 ment of the reproductive organs, and if the above reasoning holds good 

 we should expect that the smaller body-height and correlated parts are 

 accompanied by less developed reproductive organs. In other words, the 

 selection practised by man, " natural " in a sense, would have the effect of 

 reducing the size of the reproductive organs, and in order that the balance 

 of the organism may be maintained, the other organs vary concomitantly. 



* We might take the work of Bumpus on the sparrow (Biol. Lectures, Woods Holl, 

 Boston, 1898) as direct evidence in favour of natural selection. Weldon's conclusions 

 with regard to Carcinus mcenas are not so evident. On the other hand, although we find 

 many definite examples in fa your of the direct action of the environment in botany, there 

 seems to he only one in zoology — viz., the change of Artemia mlina to Artemia 

 milhausenii. 



