1895.] LUNGS OF SNAKES, AMPHISBiENrDJE, ETC. 711 



nearly all air-breathers, but that this only leads to inequality when 

 some secondary cause, such as the acquisition of a slender snake- 

 like habit of body (or in mammals some other cause, see § VII.), 

 is superadded. Moreover, it would appear that in some cases (as 

 in most Snakes), the inequality once started, the replacement of 

 paired lungs by one larger one lias in its turn led to a further dis- 

 placement of the alimentary canal and other organs. 



While thus suggesting an order of priority for correlated 

 modifications, the writer does not lose sight of the fact that these 

 modifications have all arisen under the supervision of Natural 

 Selection, and that the safest and most philosophical course is 

 simply to say that the aggregate of modifications are in some way 

 more or less advantageous. 



6. The question occurred to me whether the complete or partial 

 suppression of the right lung peculiar to Amphisbajnidse might 

 serve to tell us anything as to the stage in their evolution at 

 which the Amphisbsenidse branched off from the stock common to 

 them and other Lizards — whether, for instance, it might indicate 

 that they branched off" before their common ancestors had acquired 

 lungs, at a time, therefore, when perhaps the respective ancestors of 

 existing Lacertilia and Amphibia had diverged comparatively little 



However, on consideration it seems clear that the facts here 

 recorded do not by themselves prove any such thing, and that they 

 are not by themselves inconsistent with a considerably later 

 separation of the Amphisbaenians. 



7. This peculiarity of the Amphisbaenian lungs is for the pre- 

 sent, then, but one added to the list of the peculiarities of these 

 very interesting animals ; but the fact that (so far I have been able 

 to ascertain) no other vertebrate has the right lung suppressed, 

 suggests that this at first sight unimportant character may be 

 found to be correlated with some other character the significance 

 and importance of which may be more obvious. 



IX. Bibliography \ 



(1) Che. Litd, Nitzsch. De Eespiratione Animalium, p. 13. 



Vitebergaj, 1808. 



(2) J. F. Meckel. " TJeber die Eespiration der Reptilien. " 



Deutsches Archiv fiir die Physiologie, Bd. iv. pp. 60-89 

 [especially p. 84] and plate 2 [of which the explanation is 

 given at the end of Heft 1, pp. 162-164]. Halle, 1818. 



(3) J. F. Meckel. System der vergleichenden Anatomie, Bd. vi. 



pp. 257-262 [especially pp. 257 and 260]. Halle, 1833. 



(4) Legons d'Anatomie comparee de Georges Cuvier, redigees et 



publiees par Gr. L. Duvernoy. 2nd ed., torn. vii. pp. 19- 

 163. Paris, 1840. 



1 Milne-Edwards (6) refers to a separate paper by Lereboullet entitled 

 ' Anatomie comparee de I'appareil respiratoire.' None of the London Libraries 

 accessible to me possess a copy of this paper ; so I have not been able to see it, 

 and consequently do not put it on the list, but it is possibly quite as worthy 

 of a place there as some of the others. 



