Chap. XXV. CORRELATED VARIABILITY. 321 



in the animal kingdom ; and Moqnin-Tandon 2 remarks, that 

 as with plants the axis cannot become monstrous without in 

 some way affecting the organs subsequently produced from it, so 

 axial anomalies are almost always accompanied by deviations of 

 structure in the appended parts. We shall presently see that 

 with short-muzzled races of the clog 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 very careful not to extend this view too 

 far, for, during the normal course of development, certain members 

 in the same group of animals are known to pass through an 

 extraordinary course of change, whilst other and closely allied 

 members 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 particular 

 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 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 numerous tail-feathers, and the caudal 

 vertebrae are increased in size and number. Carriers have 

 been selected for length of 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 scutellae 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-sized than in small-sized breeds of cattle. With our 

 highly-improved animals of all kinds the period of maturity 

 has advanced, both with respect to the full growth of the body 

 and the period of reproduction; and, in correspondence with 

 this, the teeth are now developed earlier than formerly, so that, 



2 < Elements tie Teratologie Veg.,' 1841, p. 113. 

 VOL. II. 



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