INTEODUCTION 



9 



arities are the smooth ridge behind the eyes and the small 

 number of the fin-rays. 



In other respects these three species of fish are much 

 alike. There are differences in their habits and life-histories. 

 The flounder lives in estuaries and rivers, only descending to 

 the sea in the spawning season. The plaice and dab are 

 almost invariably found together. The plaice feeds mostly on 

 molluscs, the dab chiefly on Crustacea, worms, and echino- 

 derms. The young plaice congregate near shore, while young 

 dabs are found at various depths. The plaice begins to spawn 

 earlier in the year than the dab. 



Now it is quite impossible, at any rate up to the present 

 time, to find the slightest indication that the specific char- 

 acters of these three species are useful in relation to the 

 slight differences of life-history. We cannot find that the 

 rough tubercles of the flounder are useful to it because it lives 

 in rivers or for any other reason ; we know of no advantage 

 which the plaice derives from its red spots, its smooth scales, 

 or the bony tubercles on its head. Nor can we find any 

 indication that these peculiarities are correlated with adaptive 

 differences. The only adaptive difference at present clear is 

 that the plaice has blunter teeth in its throat than the other 

 species, and that these are suitable for crushing the shells of 

 the bivalves on which the plaice feeds. But we know of no 

 connection between this and the other characters. The 

 theory, then, that these specific peculiarities are due to the 

 natural selection of indefinite variations is unsupported by 

 any evidence. 



How then are we to explain such specific characters ? It 

 seems to me we are forced to regard them as the necessary 

 consequences of growth and the conditions of life. It is 

 evident enough that differences of habit and the extension of 

 a species into different regions will necessarily lead to its 

 subdivision into groups between which little or no inter- 



