^%l'''] proceedings of the national museum. 109 



If "iclithjizatiou" is in some dogreo a result of couditious found in 

 the tropics, we may expect to find a less degree of specialization in the 

 restricted and often unfavorable conditions which prevail in the fresh 

 waters, in the cold and exclusion of the Polar Seas, and especially in 

 the monotony, darkness, and cold of the oceanic abysses where light 

 can not penetrate and where the temperature scarcely rises above the 

 freezing point. 



An important factor in "ichthyization" is the reduction of the num- 

 ber of segments or vertebrie, and a proportionate increase in the size and 

 comi)lexity of the individual segment and its appendages. 



If the causes producing this change are still in operation, we should 

 naturally expect that in cold water, deep water, dark water, fresh 

 waters, and in the waters of a past geological epoch the process would 

 ibe less complete and the numbers of vertebriTj would be larger. 



And this, in a general way, is precisely what we find in the examina- 

 tion of skeletons of a large series of fishes. 



If this view is correct, we have a possible theory of the reduction in 

 numbers of vertebne as we approach tlie equator. It should, moreover, 

 not surprise us to encounter various modifications and exceptions, for 

 we know little of the habits and scarcely anything of the past history 

 of great numbers of species. The present characters of species may 

 depend on occurrences in the past concerning which even guesses are 

 impossible. 



HISTORICAL SKETCH. 



Gilnther, 1862. — The earliest observation on record in reference to 

 the subject in question was made by Dr. Albert Giinther. He noted 

 that among the Labridtv, the species of temperate waters had more ver- 

 tebra; than those of the tropics. He says : * 



lu thoae genera oi Lahridw whicli are composed entirely or for the greater part of 

 bropical species the vertebral colinuu is composed of tweuty-four vertebne, whilst 

 ihose which are chiefly confined to the temperate seas of the northern and southern 

 lemispheres have that number increased in the abdominal and caudal x>ortiou8. 



Gill, 18G3. — Shortly after, in a review of Dr. Giinther's work on the 

 iLabroidSjt Dr. Theodore Gdl showed that this generalization was not 

 jonflned to the Labroids alone, but that " it may also be extended to 

 )ther families. * * * This generalization is applicable to the rep- 

 ■esentatives of Acanthopterygian | families generally, and can be con- 

 sidered in connection with the predominance of true Malacopterygian§ 

 ishes in uorthern waters, fishes in which the increase in the number 

 )f vertebne is a normal feature." 



Gill, 1864. — Later, II Dr. Gill remarked that the increase in the num- 



* Catalogue of the Fishes of the British Museum, vol. iv, p. 65. 



t On the Labroids of the Western Coast of North America, Proc. Ac. Nat. So., Phila., 

 863, p. 221. 

 j t Spiny-rayed. 



§ Soft-rayed ; here including the Anacanthine fishes. 

 1 II Proceedings Academy Natural Science, Phila., 1864, 147. 



