2 Dr. C. F. Liitken on the Changes of Form in Fishes 



forms, especially pelagic, which, during a long series of years 

 and by means of well-directed and persevering eiforts, have 

 been collected by Danish naturalists and by officers of the 

 Danish royal and commercial navies. Of course, whenever 

 I have thought it necessary for completing ray investigations 

 and making comparisons with analogous cases, I have also 

 studied the changes produced by age in non-pelagic fishes ; 

 iind equally, of course, the criticism of the genera and species 

 to which this study has given rise has led to discussions and 

 digressions of various nature, as also to the creation of some 

 new species and genera. It follows that, while this memoir 

 is especially a contribution to the knowledge of the pelagic 

 ichthyological fauna, particularly of the intertropical Atlantic, 

 it is also indirectly a contribution to that of the ichthyology 

 of the deeper strata of the high seas ; for the inhabitants of 

 these depths, in the first phases of their development, very 

 frequently ascend, especially during the night, into the 

 warmer strata of the surface ; and they are then taken in the 

 net ; or, when they are a little older, we meet with them in the 

 stomachs of dolphins, or of voracious fishes such as the dora- 

 dos, bonitos, albacores, barracoutas, sharks, &c. But the 

 principal object of this memoir is to call attention to what I 

 call the hemimetamorphoses of fishes, a phenomenon of which 

 the pelagic fishes in particular present so many remarkable 

 examples. In adopting this expression it is not, however, 

 my intention to introduce into science a new notion or a new 

 term ; I employ it solely to characterize briefly the changes 

 which are produced during growth and development, which 

 in many cases are so considerable that they have led to the 

 complete misunderstanding of the genus, nay, often even of 

 the family to which the young individuals belong. I hope 

 by this means to profit science by a series of rectifications 

 consisting in great part in the reduction of genera and species 

 which are based only upon young, transitory forms. In many 

 cases I shall thus be led simply to confirm or extend the ob- 

 servations of my predecessors, in other cases to reject or 

 rectify conjectures or combinations which are destitute of 

 foundation. One of the consequences of a work of this 

 nature may perhaps also be that, in future, we shall proceed 

 with more circumspection and critical judgment in esta- 

 blishing new species and genera, considering the possibility 

 that the differences which may be recognized are due solely 

 to differences of age. The mistake has only too often been 

 made of regarding the latter as specific or generic differences ; 

 and this, in general, has rendered the estimation of the true 

 distinctive characters all the more difficult. 



