94 Heredity. 



more likely to cause death than to cause modification. 

 The features of microcophalous idiots show us, however, 

 that the shape of the skull and of the face is only due, 

 in part, to heredity, and is, in part at least, due to the* 

 size and shape of the brain. In lop-eared rabbits the 

 whole conformation of the skull is altered by the 

 mechanical pressure of the drooping ears, and it is stated 

 that certain monstrosities in the shape of snail-shells are 

 due to the arrested development of the reproductive 

 organs. Moquin-Tanden remarks that with plants the 

 axis cannot become monstrous without in some way 

 affecting the organs subsequently produced from it. 



We can see, from tlie study of domesticated pigeons, 

 that an increase or a decrease in certain organs is a direct 

 cause of modification in other parts. Pouter pigeons 

 have been selected for length of bod}^ and the establish- 

 ment of a lon2:-bodied race has increased the number of 

 their vertebra and the breadth of their ribs. Tumblers 

 have been selected for their small size, and the number 

 of ribs and of primary Aving-feathers has thus been 

 reduced. Fantails have been selected for their large 

 widely-expanded tails, Avith numerous tail-feathers, and 

 the size and number of the caudal veitebi-a? have thus 

 been increased, and the selection of long-beaked carriers 

 has increased the length of their tongues. Cline states that 

 the skull of a ram with horns weighs four times as much 

 as that of a hornless ram of the same age, and Youatt 

 states that in hornless cattle the frontal bones are 

 materially diminished in breadth towards the poll, and 

 the cavities between the bony plates are not so deep, nor 

 do thev extend beyond the f rentals. 



The kidneys of different birds differ much in size, and 

 St. Ange believes that this is determined by the size of 

 the pelvis. 



