190 AGE OF THE DOMESTIC ANIMALS. 



" The question now arises as to what may be regarded as 

 the cause of this early dentition in modern pigs. We know that 

 our present method of feeding causes a rapid development of 

 the whole body, including, of course, the head. As the teeth 

 could not possibly grow without a corresponding growth of the 

 jaws that produce them, we must conclude that the develop- 

 ment of the skull is the primary cause or driving force in their 

 development. Unconsciously, the modern feeder has produced 

 here some highly interesting facts instructive to natural science 

 at large. 



" Hitherto zoologists have been of the opinion that the form 

 of skull of a fixed species is unchangeable from generation 

 to generation, — we may say for thousands of years. This is cor- 

 rect as long as we think of individuals raised in the freedom of 

 nature and under natural and similar circumstances. But 

 domestication, with its forced feeding and breeding for various 

 demands, has brought about unexpected changes in many re- 

 spects, and it is now evident that the form of skull does not rest 

 merely upon heredity. Only a predisposition to a certain form 

 of skull is transferable from parents to their offsprings, but 

 whether exactly the same form Avill be transmitted depends to a 

 greater extent upon the nutrition, and but little less upon the 

 employment of the muscles of the head and neck. It is not 

 only important that the nourishment be abundant and well 

 selected, but it is also necessary that the individual be in a 

 healthy condition, and his digestive apparatus in such working- 

 order as to be able to utilize the offered food equally well. 

 This is plainly seen by comparing skulls from animals which 

 were healthy and arowing vigorously with those which received 

 the same advantages of nutrition, but were suffering with a 

 chronic disease. Continued weakness, caused either by disease 

 or insufficient food, produces a long, slender skull, while tlie 

 skull from a strong pig shows a remarkable expansion in its 

 latitude and altitude. 



" The following reproductions, taken from originals in the 

 agricultural museum at Berlin, will illustrate this point : — 



