588 Studies in the Theory of Descent. 



I cannot agree with my friend Professor Haeckel 

 when he occasionally designates the reversion of 



such characters or stages (/*. e. complexes of characters) as are 

 no longer preserved in the ontogeny, we cannot thus designate 

 the present arrest of the Axolotl at the perennibranchiate stage. 

 Such a restriction of the word, however, appears to me but 

 little desirable, since the process is identical in both cases, /. e. 

 it depends upon the same law of heredity, in accordance with 

 which a condition formerly occurring as a phyletic stage 

 suddenly reappears through purely internal processes. It is 

 true that the reversion is not complete, i. e. the present 

 sexually mature Axolotl does not correspond in all details 

 with its perennibranchiate ancestors. Since Wiedersheim has 

 shown that the existing Axolotl possesses an intermaxillary 

 gland, this can be safely asserted. This gland occurs only in 

 land Amphibians, and therefore originated with the Amblys- 

 toma form, afterwards becoming transferred secondarily to 

 the larval stage. Nevertheless, the present Axolotl must 

 resemble its perennibranchiate ancestors in most other charac- 

 ters, and we should be the more entitled to speak of a rever- 

 sion to the perennibranchiate stage as we speak also of the 

 reversion of single characters. To this must be added that 

 the Axolotl does not correspond exactly with an Amblystoma 

 larva, since Wiedersheim has shown that the space for the 

 intermaxillary gland is present, but that the gland itself is 

 confined to a few tubes which do not by any means fill up 

 this space. (" Das Kopfskelet der Urodelen." Morph. Jahr- 

 buch, vol. iii. p. 149). By the expression " arrested growth " 

 not much is said, if at the same time the cause of the arrest 

 is left unstated. But what can be the cause why the whole 

 organization remains stationary at the perennibranchiate stage, 

 the sexual organs only undergoing further development ? 

 Surely only that law or force of heredity known by its effects, 

 but obscure with respect to its causes, through which old 

 phyletic stages sometimes suddenly reappear, or in other words, 

 that power through which reversion takes place. It must not 

 be forgotten that all these cases of " larval reproduction " in 



