362 ON CHANGE OF MEXICAN AXOLOTL TO AN AMBLYSTOMA. 



that oa more careful consideration much of the paradox which seems to 

 lie in this view will disappear. 



Before all else, it is to be considered that the phyletic development 

 of the species need by no means have always gone directly forward. 

 We have examples enough of development backward, although iu 

 rather a different sense, as iu parasites and such forms, which have sunk 

 from free locomotion to the sedentary mode of life. 1 do not ignore the 

 difference which exists between this sort of development backward by 

 the pining away of certain organs or systems of organs, and proper re- 

 version. The latter is the return to an anvnal form already once existent j 

 but in the former case, in spite of all simplification of the organization, 

 something wholly new is always formed. But I am able to see nothing- 

 absurd as to principle in the supposition that a proper reversion, also, 

 be it of a whole species or of the individuals of a species within a cer- 

 tain habitat, is thought oiaspossiblej and I do not for the present demand 

 a further concession. Why, for example, should it be so entirely in- 

 credible that the Axolotl, in a time long past, was adapted to living on 

 the land, that it gradually, through direct and indirect operation of al- 

 tered conditions of life, had acquired the salamander form, but that 

 later, by a new change of the circumstances of life unfavorable to its pres- 

 ent organization, it has again fallen back into the old form or one lying 

 near it ? At any rate, such a supposition contains nothing which would 

 stand iu contradiction with well-known facts, but can be sustained iu 

 several ways; and finally, it recommends itself by furnishing, at least 

 in my opinion, the only possible explanation of the facts before us. 



The above-mentioned existence of a whole group of species of Ambly- 

 stoma shows at once that species of Siredun can rise to the salamander 

 form, and can regularly propagate iu that form ; and further, that this 

 phyletic advance has already actually taken place in very many species. 

 But that a sinking back from this higher stage of development to the 

 lower can also take place, several observations on our water salaman- 

 ders show. 



It is well known that Tritons under some circumstances become, as it 

 is usually expressed, " sexually mature in the larval state." 



In the year 18G1 De Filippi* found in a swamp by Lake Maggiore 

 fifty Tritons, of which only two showed the structure of the full grown 

 water salamander; but all the others still had their gills, yet corre- 

 sponded to mature animals in bodily size and iu the development of the 

 sexual organs, and that iu both sexes. 



Filippi determined the fact that these "sexually mature larvae'' did 

 not merely resemble larvjB externally by the possession of gills, but that 

 they also presented all the other anatomical marks of the larvae ; that 

 is, the characteristic clusters of palatal teeth standing on both sides, 

 instead of the later simple row, and a vertebral column, which still has 

 the Chorda dorsalis running through its whole length. 



* Sulla larva del Triton alpestris, Archivio per la Zoologia, 1861. 



