Chap. V. CORRELATION OF GROWTH. 177 



furculum of the short-faced, small-sized Tumbler (a), whereas the 

 divergence in c equals that in a rock-pigeon, or in the Pouter (d), 

 though the latter is a much larger bird. The extremities of the furcu- 

 lum, where articulated to the coracoids, vary considerably in outline. 

 In the sternum the differences in form are slight, except in the 

 size and outline of the perforations, which, both in the larger and 

 lesser sized breeds, are sometimes small. These perforations, also, 

 are sometimes either nearly circular, or elongated as is often the 

 case with Carriers. The posterior perforations occasionally are not 

 complete, being left open posteriorly. The marginal apophyses 

 forming the anterior perforations vary greatly in development. 

 The degree of convexity of the posterior part of the sternum differs 

 much, being sometimes almost perfectly flat. The manubrium is 

 rather more prominent in some individuals than in others, and the 

 pore immediately under it varies greatly in size. 



Correlation of Growth. — By this term I mean that the whole 

 organisation is so connected, that when one part varies, other 

 parts vary ; but which of two correlated variations ought to be 

 looked at as the cause and which as the effect, or whether both 

 result from some common cause, we can seldom or never tell. 

 The point of interest for us is that, when fanciers, by the con- 

 tinued selection of slight variations, have largely modified one 

 part, they often unintentionally produce other modifications. 

 For instance, the beak is readily acted on by selection, and, 

 with its increased or diminished length, the tongue increases 

 or diminishes, but not in due proportion ; for, in a Barb and 

 Short-faced Tumbler, both of which have very short beaks, the 

 tongue, taking the rock-pigeon as the standard of comparison, 

 was proportionally not shortened enough, whilst in two 

 Carriers and in a Eunt the tongue, proportionally with the 

 beak, was not lengthened enough, thus, in a first-rate English 

 Carrier, in which the beak from the tip to the feathered base 

 was exactly thrice as long as in a first-rate Short-faced 

 Tumbler, the tongue was only a little more than twice as 

 long. But the tongue varies in length independently of the 

 beak : thus in a Carrier with a beak 1-2 inch in length, the 

 tongue was '67 in length : whilst in a Runt which equalled 

 the Carrier in length of body and in stretch of wings from 

 tip to tip, the beak was *92 whilst the tongue was *73 of an 

 inch in length, so that the tongue was actually longer than 

 in the carrier with its long beak. The tongue of the Runt 

 was also very broad at the root. Of two Runts, one had its 

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