chap, xix.] LATERAL HOMCEOSIS : COMMENT. 473 



Linear Series. In the Flat-fish the right side and the left have been 

 differentiated on different lines, as the several appendages of an Ar- 

 thropod have been, but on occasion the one may suddenly take up all 

 or some of the characters, whether colour, tubercles or otherwise, in 

 the state to which they have been separately evolved in the other. 



What may be the cause leading to this discontinuous change we do 

 not know. That it is often associated with a delay in the change of 

 position of the eye of the " blind " side seems clear from the frequent 

 detachment of the dorsal fin in these cases. But it should be borne in 

 mind that even in such examples the eye may still eventually get to 

 its normal place, though probably it was delayed in the process and so 

 led to detachment of the fin. Taken with the fact that the young 

 " double " turbots swim on edge longer than the normals it must be 

 concluded that the bilateral symmetry of colour is associated with 

 reluctance or delay in the assumption of the asymmetrical state, but 

 more than this cannot be affirmed. 



I do not urge that the same reasoning should be applied in other 

 cases, but the possibility must be remembered. In the Narwhal, for 

 instance, it is perhaps unlikely that there was ever an ancestor which 

 had two tusks developed to the extent now reached by the left tusk of 

 the male ; but if there ever were any such form, it is hard indeed to 

 suppose that it could have been connected with the present species 

 by a series of successive normals in which the right tusk gradually 

 diminished while the left was of its present size. On the whole it 

 seems more likely that when the right tusk now develops to be as 

 long as the left, it is taking up at one step the state to which the left 

 has been separately evolved. 



However this may be, the fact that such Horuceosis is possible 

 should be kept in view in considering the meaning of such cases as that 

 of a Tornaria with two water-pores. For while on the one hand we 

 may suppose that Balanoglossus kupfferi with its normal pair of water- 

 pores is the primitive state and that the varying Tornaria is a revers- 

 ion, on the other hand B. kupfferi may be a form that has arisen by a 

 Homoeotic variation from the one-pored form, and of this variation 

 Balanoglossus No. 725 may be a contemporary illustration 1 . 



1 The following interesting example of a similar Variation has appeared since 

 these pages were set up. Eledone cirrhosa : specimen having not only the third 

 left arm developed as a hectocotylus, as usual, but the third right arm also. The 

 right had 57, the left 66 suckers, but otherwise they were alike. Appellof, A., 

 Bergens Museums Aarbog, 1893, p. 14. 



