BIRDS' EGGS 245 



recalled that bats are usually uniparous. One may be 

 reminded of the pigeon's heavy crop-full of Indian corn or 

 grain, or of the eagle lifting the rabbit or something 

 heavier, and it is clear that once a bird has got a-going 

 a little additional weight is neither here nor there. On 

 the other hand, the getting a-going is often rather strenuous 

 and every additional ounce tells. Even the pigeon v^^ith 

 its enormous pectoral muscles becomes terribly fatigued if 

 it is forced to rise from the ground many times in rapid 

 succession. It is well known that some voracious sea-birds 

 may eat so much at a meal that they cannot fly at all for a 

 considerable time. Therefore the reduction of weight in a 

 flying bird cannot be dismissed as of no importance in the 

 struggle for existence. 



Perhaps there is something more important than the 

 weight of an additional ovary and an additional oviducal 

 egg, namely the reduction of the space available for the air- 

 sacs. We admit, however, that in most birds a ventral 

 sagittal partition divides the abdominal cavity into two 

 distinct compartments ; that the reproductive activity of 

 birds is sharply punctuated ; and that in many a bird outside 

 of the breeding season the reproductive system has dwindled 

 into insignificance. Yet, on general biological grounds, it is 

 quite possible that the selection which determined the survival 

 of females with one-sided development of the reproductive 

 system operated during the breeding season. One must not 

 think of each new improvement being patented separately ; 

 many siftings go on at the same time ; it is the best tout- 

 ensemble during the reproductive period that counts for the 

 race. 



Descending from theory to fact, we should notice that 

 the unilateral development is not absolute. Max Kollman 

 reports (1919) that in six cases among falcons (four of 

 Accipiter nisus (sparrow hawk) one of Astiir palumbarius 

 (gos-hawk) and one of Tinnwicv.liis tinnunculus (kestrel)) 

 paired ovaries were found. Yet in these cases the right 

 oviduct remained quite vestigial, which is a little puzzling. 

 In a duck observed by Chappellier there were two ovaries 



