VARIATION 361 



the spinulation greater in plaice of the northern part of the 

 North Sea, than in those of the southern part. The differences 

 are more distinct when we compare plaice from the Baltic with 

 those from the North Sea, the number of fin-rays being less, 

 and the spinulation greater in the former than in the latter. 



Such slight differences in the characters of samples from 

 different localities are of very frequent occurrence in species 

 both of animals and plants, and they give rise to problems 

 which are by no means fully solved. They seem to be due to 

 the difference of conditions in the localities, and there is some 

 evidence that they are due to the direct influence on the indi- 

 vidual and are not constant and hereditary. But it is difficult 

 to see how the conditions should affect the characters in ques- 

 tion, why for example the conditions of the Baltic should pro- 

 duce a reduction of fin-rays and greater spinulation. We cannot 

 trace a general influence as in the case of the relation between 

 light and pigmentation, nor have we any evidence that the 

 differences are due to mechanical or functional stimulation, or 

 to selection. With regard to the last factor it is impossible to 

 believe that small variations in many of the specific characters 

 we are considering could be selected by their direct utility to 

 the individual, because the characters themselves, so far as we 

 can see, have no utility. We have no evidence that the tuber- 

 cles on the head, or the red spots, are of any benefit to the 

 plaice, although they serve to distinguish that species from the 

 flounder ; the spinules may be due to some kind of mechanical 

 stimulation involved in the habits of the males, but they can 

 scarcely be of any use. On the other hand, the fins are im- 

 portant functional organs, and a difference in their extent in- 

 volves a difference in the number of fin-rays. The scales of 

 Teleostei afford some of the most important specific characters, 

 and these characters exhibit continuous variations, but there is no 

 sound reason for the belief that the scales are of any benefit to 

 the fish at all. They are rudimentary in the plaice, represented 

 in the flounder and the turbot only by scattered tubercles ; 

 the brill has regular spiny scales, and in these and a vast num- 

 ber of other cases it would, as far as we can see, make no differ- 

 ence to the survival of the fish if the scales or tubercles were 

 altogether absent. 



Mr. Garstang has investigated the variations of the mackerel, 



