LEAVES FROM MY NOTE-BOOK. 53 



nevertheless, when paired with the common duck, he transmits to 

 his female offspring a strong quacking tendency." ''' 



Correlation of Growth. 



Speaking of the correlation of variations, Darwin describes, at 

 great length, the extraordinary osteological changes in the skulls of 

 crested fowls ; beginning in the black boned silk fowl with a very 

 small crest, and the skull penetrated only by a few minute orifices ; 

 and culminating in fowls which, with a largely developed crest, 

 have a largely protuberant skull, perforated by a multitude of 

 irregular open spaces, and containing a portion of the brain sub- 

 stance. He adds, " There can be no doubt that in former times 

 the breeders of Polish fowls attended solely to the crest, and not 

 to the skull ; nevertheless, by increasing the crest, he has uninten- 

 tionally made the skull prominent to an astonishing degree ; and, 

 through correlation of growth, he has, at the same time, affected 

 the form and relative connection of the premaxillary and nasal 

 bones, the shape of the orifice of the nose, the breadth of the 

 frontal bones, the shape of tlie post-lateral processes on the frontal 

 and squamosal bones, the direction of the axis of the long cavity 

 of the ear, and lastly, the internal configuration of the whole 

 skull, together with the shape of the brain." 



Darwin gives numerous instances of breeds of animals and 

 varieties of plants originating from "sports," and uses the expression 

 that the variations " arose suddenly." Paleontology leads one to 

 infer that differentiation of species has arisen by slow and almost 

 imperceptible degrees, and this seems to be the usual course of 

 evolution ; but, on the other hand, we have a great weight of first- 

 hand evidence as to the origination of fresh breeds by sudden 

 variations. 



I will take one case, that of the Black Peacock. Sir R. Heron 

 states that this breed suddenly appeared, within his memory, in 

 Lord Brownlow's large stock of pied, white, and common peacocks, 

 and in Mr. Thornton's stock of common and pied peacocks. It 

 is remarkable that in these two latter instances the black-shouldered 

 kind, though a smaller and weaker bird, increased " to the extinc- 

 tion of the previously existing breed." No less than seven well 



* Vol. I., pp. 295 — 6. Ibid., p. 279. 



