Peculiar Clutch of Blackbird's Eggs. 95 



X. — Note on a Peculiar Clutch of Blackbird's Eggs and some other 

 Abnormalities. By Professor J. Arthur Thomson, M.A. 



(Read 28th February 1910. Received 28th February 1910.) 



That the eggs of birds show considerable variability — in size, shape, and 

 colouring — is, of course, very well known, but I thought it might be of use to 

 record an interesting case of variation — especially in size — in a clutch of 

 Blackbird's eggs. I owe the clutch and the data as to the order in which the 

 eggs were laid to my friend Mr R. Hay Fenton, who not long ago very 

 generously presented to the Natural History Museum of Aberdeen University 

 his remarkably fine and almost complete collection of the eggs of British 

 birds. 



There are four eggs in this Blackbird's clutch, which have the following 

 dimensions — 



(i.) 38 x 24 mm. 

 (ii.) 33 x 23 mm. 

 (iii.) 28 x 22-5 mm. 

 (iv.) 27 x 21 mm. 



The largest egg (i.) was laid first, but it did not develop ; the next 

 largest (ii.) followed and was fertile ; the third (iii.) was near the normal 

 size ; the smallest egg (iv.) was laid last. The third and fourth were quite 

 fresh when the clutch was taken. 



In his great work on the eggs of the birds of Europe, Dresser notes that 

 the eggs of the Blackbird vary from 28 x 2133 mm. to 32 x 2235 mm., and 

 quotes as an average of 48 eggs, 28 x 21 mm. ; so that the interest of the 

 clutch exhibited is that the first egg is far above the average, and that 

 the abnormality goes on decreasing egg after egg. There is a progressive 

 variation in a minus direction. 



I do not wish to make too much of a little thing, but it is of interest to 

 compare a case like this with others where we see at work the mysterious 

 regulative tendency which is characteristic of living creatures. Thus 

 Raymond Pearl recently described a case {Joum. Exper. Zool., vi., 1909, pp. 

 339-351, 1 pi.) in which the first egg laid by a pullet was very abnormal in 

 shape — elongated ovate pyriform. In the subsequent eggs laid there was a 

 quite gradual change of shape, which was regulative in character, until 

 finally the eggs were quite normal. 



Numerous abnormalities in the eggs of the common fowl have been 

 recorded, and the collection now shown illustrates not a few. It is an 



