328 DR. GADOW ON THE ANATOMY OF PTEROCLES. [Mar. 21, 



exist in our common Pigeon, and thus they are in contrast with those 

 birds in which caeca are altogether wanting, like Woodpeckers, Parrots, 

 and others. Garrod likewise included the Passerine birds amongst the 

 Menotyphla (to use a Hseckelian term for animals possessing caeca). 

 Now I think this is not correct ; and we must consider this matter 

 a little further. Garrod himself came to the conclusion that the an- 

 cestral bird-stock did possess caeca ; as this is undoubtedly true, it 

 follows that all those birds which are now found without caeca 

 must have lost them, either phylogenetically or even during their 

 ontogenetic development. In fact we see, in embryos of such birds 

 as have when adult only very small quite rudimentary caeca, 

 that these organs are, in the embryo, just as well developed as in 

 birds with long caeca ; but these caeca, in a Pigeon for instance, do 

 not grow any further. They are in early life stopped in their deve- 

 lopment, and thus remain in a rudimentary state. Again, in all those 

 birds which are completely devoid of caeca the tendency to suppress 

 these organs is simply carried out to the extreme. We cannot, 

 therefore, group the birds into birds with caeca and birds without 

 caeca ; and this is especially wrong, as there exist many birds which, 

 although apparently allied to each other, differ greatly in the pre- 

 sence or absence of caeca. 



If we want to take the caeca into consideration at all, we must 

 take another point of view : that is, are the caeca of any use to the 

 birds in question or are they not ? Now, apparently, in all birds 

 which have well-developed caeca they are useful, although we must 

 confess that we do not know in what way. Again, in birds with 

 very small caeca, where these organs are simply vermiform-like pro- 

 cesses, and which never contain any chyle in their extremely small 

 lumen, they cannot have any physiological function, else they would 

 not have been suppressed. 



No doubt in some cases, in which they are not quite aborted, as 

 for instance in the Crows and in our common Pigeon, the glands in 

 their walls may still produce some secretion, which then may be 

 made the best of. But this is one of the cases in which rudimen- 

 tary organs are not completely stopped in their functions although 

 they are useless, simply because the animal hitherto has not been 

 able to get rid of them entirely : thus, for instance, the appendix 

 vermiformis of man, or another example still more striking, our 

 thymus gland, which, although a gland, is now without a duct, and 

 thus rather a paradox. 



But to return to our question. It is clear that birds with rudi- 

 mentary caeca have to be grouped together with lipotyphlous birds, 

 i. e. birds which have lost these organs. 



The great development of the caeca therefore constitutes a consider- 

 able difference between the Pteroclidae and the Columbidae, as the 

 former and the Gallinacei are decidedly menotyphlous and the 

 Columbse lipotyphlous. 



In the Gallinacei the whole digestive tract always forms four very 

 distinct loops : the duodenal one is the first ; the next two loops are 

 formed by the ileum ; in birds which, like Perdix, have a compara- 



