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THE AMERICAN NATURALIST [Vol. LV 



not take place before the eyes of the host metamorphose. 

 On the other hand, eyes of young hirviP, if they are 

 grafted to old larvae, can be made to metamorphose earlier 

 than they would under normal conditions (12). 



It has been said that the thyroid substance does not 

 actually produce new characters, but merely accelerates 

 the rate of their development which is predetermined by 

 heredity. There can be little doubt, however, that the 

 advance of the higher vertebrates from an aquatic stage, 

 with open gill slits and internal or external gills, and in 

 particular all the characters distinguishing the terres- 

 trial amphibian from the aquatic larva, could not have 

 developed if the thyroid apparatus had not attained, at 

 some evolutionary stage of the amphibians, its present 

 function. For the benefit of those who might think that 

 the relatively short time (about years) of observation 

 in Allen's and Hoskins's experiments does not justify this 

 statement, I may refer to the Texan cave salamander, 

 TypJilomolge rathhuni which illustrates in a most vivid 

 manner the ei¥ect of the absence of the thyroid gland. 

 This salamander never develops beyond the larval stage, 

 retaining permanently its external gills and other larval 

 organs. An examination of the endocrine system of this 

 animal was made by P]merson (1.'5) ; it rcNcalcd the com- 

 plete absence of the thyroid gland. It is worth while to 

 mention briefly another interesting condition ()i)st i\'('(l iii 

 this animal, namely the almost complete lack of i)igni('nt, 

 a condition somewhat similar to that observed by Smith 

 and by Alien in liypophysectomized tadpoles, and the 

 highly atrophied state of the eyes. Ti/pJdomolf/e is a 

 white, blind salamander, 'i'liese latter peculiarities have 

 been attributed frequently to the absence of light in the 

 caves, a tlieory which at first seems very plausible. It 

 would not be surprising, however, if some dav these char- 

 acters ^houhl be found to h," thr r.'Mih nf ..udocrine dis- 

 turh;.nc(-. Simihir to T uphlnun,' i,, nil the charac- 

 teristics mentioned above i^ a Kn rop^a n salamander, 

 Proteus (ninunicus, which inlial)its the Austrian lime- 



