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Show 708 MR. GERARD W. BUTLER ON THE [NOV. 19, The first two questions are very interesting, and I hope shortly to return to their discussion in another paper. Por the present 1 may merely say that I incline to a view similar to that suggested by Goette in 1875, namely that the lungs have arisen from paired lateral branchial pouches 1. Anyone who adopts this view will recognize a certain tendency to pairedness of the lungs as primitive. It seems, however, highly probable that lungs have arisen [from some such common anlage] independently in the different groups of vertebrates, and that we ought not to conclude that all pulmonate vertebrates are descended from a common pulmonate ancestor. To find such common ancestor we should perhaps have to go back to a time long before the first appearance of pulmonary respiration. It is thus quite conceivable, even accepting Goette's view, that in the ancestors of certain one-lunged types the branchial pouch of one side may have from the first remained rudimentary, that of the other^ side alone developing into a lung. Such a view is also quite in harmony with embryology; for in the embryos of such forms as Vipera aspis and Typhlops lumbricalis there is no trace of a second lung even in early stages. While, however, neither embryology nor the theory of homology with paired branchial pouches runs counter to the view that the ancestors of some pulmonates may from the first have had but one lung, while others had two, it seems to me that there are certain facts of comparative anatomy which are in favour of the view that in their first beginning the lungs were not only potentially but actually paired in the ancestors of many species which now have no trace of more than one. Thus, as is well known, we find cases of two species of Snake which are so alike in other respects as to be classed in the same genus, one of which has a rudiment of the left lung, while the other has no trace of such2. N o w the pesistence of the rudiment as such a definite structure in the adult, combined with the fact that the rudiment is of proportionally greater size in the embryo, suggest that it is the reduced remains of an organ which was once a functional lung. If, then, a functional lung can be reduced to a mere functionless rudiment, it seems likely, when we find two species of the same genus, one of which has such rudiment while the second has not, that in this second the reduction has but been 1 The clue to my reason for taking this view is briefly this, that I find that in the Lizard, Snake, and Bird the oesophagus becomes separated off, from behind forwards, from the anlage of the lungs and from the trachea, just as it would appear from Nestler's observations the anlage of the oesophagus is separated off from the branchial chamber in the metamorphosis of Ammocaetes into Petromyzon [Nestler, ' Arehiv fur Naturgescbichte,' Jahrg. lvi. Bd. i. pp. 100- 105]. From the best published accounts the same is true of the development of the oesophagus, lungs, and trachea of Amphibia and Mammalia. 2 Thus in m y list above Crotalus horridus has a small rudiment, while C. durissus has none ; Elaps hygeia has a rudiment, while E. fulvius has none. Similarly, in Cope's paper (7) p. 223, we have such a difference recorded in two other genera besides Crotalus, viz. in Bothrops and Ancistrodon. |