Chap. XXV. COEEELATED VAEIABILITY. 327 



with Polish fowls the tuft of feathers was probably at first 

 small ; by continued selection it became larger, and then 

 rested on a fibrous mass ; and finally, as it became still 

 larger, the skull itself became more and more protuberant 

 until it acquired its present extraordinary structure. Through 

 correlation with the protuberance of the skull, the shape 

 and even the relative connection of the premaxillary and 

 nasal bones, the shape of the orifice of the nostrils, the 

 breadth of the frontal bone, the shape of the post-lateral 

 processes of the frontal and squamosal bones, and the 

 direction of the bony cavity of the ear, have all been 

 modified. The internal configuration of the s«kull and the 

 Avhole shape of the brain have likewise been altered in a 

 truly marvellous manner. 



After this case of the Polish fowl it would be superfluous 

 to do more than refer to the details previously given on the 

 manner in which the changed form of the comb has affected 

 the skull, in various breeds of the fowl, causing by correlation 

 crests, protuberances, and depi'essions on its surface. 



With our cattle and sheep the horns stand in close con- 

 nection with the size of the skull, and with the shape of the 

 frontal bones ; thus Cline 35 found that the skull of a horned 

 ram weighed five times as much as that of a hornless ram of 

 the same age. When cattle become hornless, the frontal 

 bones are " materially diminished in breadth towards the 

 " poll ; " and the cavities between the bony plates " are not so 

 " deep, nor do they extend beyond the frontals." 36 



It may be well here to pause and observe how the effects of 

 correlated variability, of the increased use of parts, and of the 

 accumulation of so-called spontaneous variations through 

 natural selection, are in many cases inextricably commingled. 

 We may borrow an illustration from Mr. Herbert Spencer, who 

 remarks that, when the Irish elk acquired its gigantic horns, 

 weighing above one hundred pounds, numerous co-ordinated 

 changes of structure would have been indispensable, — namely, 

 a thickened skull to carry the horns; strengthened cervical 



35 ' On the Breeding of Domestic Animals,' 1829, p. 6. 



36 Youatt on Cattle, 1834, p. 283. 



