142 THE GROWTH OF GROUPS 



two distinct types which may be called the orderly type, 

 and the disorderly type. Let us consider how these 

 attributes are present among the fish. 



The first is best studied by measurement. If we 



determine the value -. ^-n X 100 in a large number of 

 length 



any kind of flat fish, such for example as the skate, we 

 shall find that the range of variation is less than ten as 

 a rule. But in the twenty-one measured specimens of 

 Malthopsis, the range of variation is as much as forty- 

 four. The actual values arranged in series are as follows 



49, 50, 51, 53, 5567, 72, 72, 73, 73, 74, 74, 74, 74, 75, 

 76, 77, 80, 8592, 93. 



Although the individuals are unfortunately few, yet 

 there are enough to show at least three types, in which 

 the mean of the values expressing the breadth of the disc 

 are approximately 51, 74, and 92. No one who has 

 examined the collection would deny that the group is 

 trimorphic as regards the breadth of the disc. The 

 individual with the value eighty-five which is inter- 

 mediate between the medium and broad is a curious 

 unsymmetrical specimen which appears to be of the 

 medium type as regards one half of the body and of the 

 broad type as regards the other. 



Now let us consider the dermal plates. As regards 

 them, the fish appear in two states, some in one state and 

 others in another. We may speak of these states as 

 characters without making any assumption as to the 

 nature of a character. The general nature of the arma- 

 ture is alike in all, that is to say it consists of soft skin 

 in which are embedded conical limpet-like plates of bone, 

 and, as mentioned already, there are three prominent 



