Chap. XXV. COEEELATED VAEIABILITY. 313 



accompanied by deviations of structure in the appended parts. 

 We shall presently see that with short-muzzled races of the 

 dog certain histological changes in the basal elements of the . 

 bones arrest their development and shorten them, and this | 

 affects the position of the subsequently developed molar teeth. 

 It is probable that certain modifications in the larvae of 

 insects would affect the structure of the mature insects. But 

 we must be careful not to extend this view too far, for 

 during the normal course of development, certain species pass 

 through an extraordinary course of change, whilst other and 

 closely allied species arrive at maturity with little change 

 of structure. 



Another simple case of correlation is that with the increased 

 or decreased dimensions of the whole body, or of any parti- 

 cular part, certain organs are increased or diminished in 

 number, or are otherwise modified. Thus pigeon-fanciers have 

 gone on selecting pouters for length of body, and we have 

 seen that their vertebrae are generally increased not only in 

 size but in number, and their ribs in breadth. Tumblers have 

 been selected for their small bodies, and their ribs and primary 

 wing-feathers are generally lessened in number. Fantails have 

 been selected for their large widely-expanded tails, with nu- 

 merous tail-feathers, and the caudal vertebrae are increased in 

 size and number. Carriers have been selected for length ot 

 beak, and their tongues have become longer, but not in strict 

 accordance with the length of beak. In this latter breed and 

 in others having large feet, the number of the scutelke on the 

 toes is greater than in the breeds with small feet. Many 

 similar cases could be given. In Germany it has been 

 observed that the period of gestation is longer in large than 

 in small breeds of cattle. With our highly-improved 

 breeds of all kinds, the periods of maturity and of repro- 

 duction have advanced with respect to the age of the 

 animal ; and, in correspondence with this, the teeth are 

 now developed earlier than formerly, so that, to the surprise 

 of agriculturists, the ancient rules for judging of the age of 

 an animal by the state of its teeth are no longer trustworthy. 3 



3 Prof. J. B. Simonds, on the Age of the Ox, Sheep, &c, quoted in ' Card, 

 Chronicle,' 1854, p. 588. 



