Chap. XXV. COEBELATED VAEIABILITY. 333 



perforated skulls. The case would probably be considered by 

 some authors as one of balancement or compensation. In the 

 chapter on Fowls, I have shown that with Polish fowls the tuft 

 of feathers was probably at first small ; by continued selection 

 it became larger, and then rested on a fleshy or fibrous mass ■ 

 and finally, as it became still larger, the skull itself became 

 more and more protuberant until it acquired its present ex- 

 traordinary structure. Through correlation with the protu- 

 berance of the skull, the shape and even the relative connexion 

 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 modi- 

 fied. The internal configuration of the skull and the whole 

 shape of the brain have likewise been altered in a truly mar- 

 vellous 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, in various 

 breeds of the fowl, has affected the skull, causing by correlation 

 crests, protuberances, and depressions on its surface. 



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

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

 frontal bones ; thus Cline 31 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." 32 



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 through natural selection of so-called spontaneous 

 variations, 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 



« 'On the Breeding of Domestic Animals,' 1829, p. G. 

 s - Youatt on Cattle, 1834, p. 283. 



