SOME FACTS ABOUT FISHES. 49 
Of course we have been unable to give more than a general 
indication of the nature of these remarkable external changes 
in this very rapid and imperfect sketch. But we must notice 
that in many cases changes occur in internal structure of 
quite as great importance as those to which we have alluded. 
Among other things, the dentition frequently undergoes most 
remarkable alterations, and this is a matter of great con- 
sequence in relation to systematic ichthyology, and especially 
to the interpretation of the fossil remains of fishes. In 
fact, we may say broadly that a great proportion of young 
fishes differ from their parents by characters which would 
be regarded as sufficient for the establishment of new genera 
and species, if occurring in adult fishes ; and hence it will 
be easily understood that, with the fragmentary acquaintance 
that we necessarily possess of the fish fauna of the high seas, 
and even of the shores of tropical countries, great numbers 
of young forms have already been described as independent 
genera and species. The two distinguished ichthyologists 
whose names were mentioned at the beginning of this article, 
and from whose writings most of its materials have been 
derived, have already indicated a great number of important 
systematic changes of this kind, which must be made in conse- 
quence of the recognition of the facts here briefly and im- 
perfectly described. But a study of these works shows that 
while considerable progress has been made, much more still 
remains to be done. The changes referred to appear to be 
most strongly marked in pelagic and deep-sea fishes, which are 
not only difficult to get, but naturally so widely distributed, 
that it is only by lucky accident, or by long- continued inves- 
tigation, that we can hope to bring together the successive 
forms of a species so as to work out its development. 
NEW SEMES, VOL. V. — NO. XVII. 
