Patten : Dimorphism in the Eggs of the House-Sparrow. 301 



that such clutches are as genuine and as sure of being laid by 

 one bird as when no dimorphism at all occurs, which at the 

 same time must be distinguished from individual variations 

 which are (except in the cases of unspotted eggs), exceedingly 

 common. In fact no two eggs are ever spotted exactly alike ; 

 they differ individually as human faces do, though both are 

 capable of being grouped into types. In the above clutch of 

 Sparrows' eggs we find two types evidently laid by the one 

 mother-bird, the egg with the clean white ground-colour and 

 more or less discreet spots, and the egg with the groiind-colour 

 diffused to a considerable extent by admixture with the fine 

 pigmental markings. 



I now proceed to describe three other sets of eggs, each set 

 being the contents of one nest. I cannot call them all clutches, 

 as from what follows I have reason to believe that in two cases 

 more than one hen-bird laid in the nests. The top row in Plate 

 XIV., fig. I, shows four eggs marked (apart from individual 

 variation) very similarly, but the discrepancy in size between a, 

 b. and c. d, is so well marked that an interesting example of 

 dimorphism in size is seen ; as this is not a very uncommon 

 occurrence, and as here the stages of incubation were fairly uni- 

 form, I am inclined to believe these four eggs form a true clutch. 



The other two sets of eggs, figured in Plate XIV., fig. i, are 

 more remarkable, and as regard markings, show the converse 

 condition to the last, and indeed in size and shape we see consider- 

 able differences also. In the bottom row m and ;;, while very 

 similar in size and markings, are so unlike 0, that at first one 

 might be inclined to think that the former were not a House- 

 Sparrow's eggs at all. I must frankly admit that I was doubt- 

 ful myself until I received a general consensus of opinion from 

 the ornithologists present at a recent meeting of the Yorkshire 

 Naturalists' Union * in favour of the House-Sparrow being 

 responsible for producing such an interesting variety of egg. 

 In regard to this set I ma\' here refer to a noteworthy 

 fact about the contents. Originally the number of eggs found 

 in the nest was five, but unfortunately two others of the same 

 type as became broken ; however, though the contents were 

 somewhat besmattered on the floor, I managed to find out that 

 fertility had ensued and that a very early stage of the embryo 

 had been reached, also that there wei'e e\-ident signs that the 



* These nggs were exhibited at the meeting- of the Vertebrate Section of 

 Zoologry, held at Leeds on Feb. iqth, igio. 



1910 Aug. I. X 



