AMERICAN ZOOLOGISTS AND EVOLUTION. 183 



few examples being known in which the creatures have lost their 

 gills and assumed the mature characters of Amblystoma, hut with 

 Siredon a change takes place with a proper change of surroundings. 



To American students we are indebted for most valuable contri- 

 butions regarding the effect of cave influences on animals living 

 within their boundaries. Looking at the cave fauna with its pecul- 

 iar assemblage of animals, it would seem that here, at least, the 

 question as to the effects of certain external influences, or the absence 

 of others in modifying structure, might be found. 



Many years ago the editors of Silliman's Journal addressed a 

 letter to Prof. Agassiz respecting the blind fishes of the Mammoth 

 Cave, and asked his opinion as to whether their peculiar structure 

 was due to their cave life, or whether they had been specially created. 

 Agassiz's ' reply is consistent with his belief. He says, " If physical 

 circumstances ever modified organized beings, it should be easily as- 

 certained here." He then expresses his conviction that " they were 

 created under the circumstances in which they now live, within the 

 limits over which they range, and with the structural peculiarities 

 which characterize them at the present day," adding frankly, how- 

 ever, that these opinions are mere inferences. 



With the contributions on cave insects by the eminent zoologist 

 Schiodte, and our own naturalists as well, we have now overwhelm- 

 ing proof that the blind fishes and numerous other cave animals are 

 marked with peculiarities impressed upon them by the unusual envi- 

 ronments to which they have been subjected. 



In a work on the animals of the Mammoth Cave, by Dr. A. S. Pack- 

 ard and Prof. Putnam, the first-named writer quotes the results of 

 Schiodte, wherein he shows the existence of twilight animals in which 

 but slight modification occurs, while in darker places the changes be- 

 come more profound. 



Dr. Packard 2 sums up the results of his work as follows : " We 

 then see that these cave animals are modified in various ways, some 

 being blind, others very hairy, others with long appendages ; all are 

 not modified in the same way in homologous organs, another argu- 

 ment in proof of their descent from ancestors whose habits varied as 

 their out-of-door allies do at present." 



Prof. E. D. Cope, 3 in an article on the fauna of Wyandotte Cave, 

 in commenting on the loss of eyes in cave animals from absence of 

 light, and consequent disuse, says that, to prove it, " we need only to 

 establish two or three propositions: 1. That there are eyed genera 

 corresponding closely in other general characters with the blind ones. 

 2. The condition of the visual organs is in some cave type vari- 

 able. 3. If the abortion of the visual organs can be shown to take 

 place coincidently with general growth to maturity, an important 



1 American Journal of Science, second series, vol. xi., p. 128. 



2 " Life in the Mammoth Cave," p. 27. 3 American Naturalist, vol. vi., p. 415. 



