28 PRINCIPLES OF BREEDING. 



teristic phenomena, which a late writer* enumerates as 

 follows : 



1. ''They are transmitted by the male as well as by 

 the female parent, and are doubly severe in the offspring 

 of parents both of which are affected by them. 



2. They develop themselves not only in the immedi- 

 ate progeny of one affected by them, but also in many 

 subsequent generations. 



3. They do not, however, always appear in each gen- 

 eration in the same form ; one disease is sometimes 

 substituted for another, analogous to it, and this again 

 after some generations becomes changed into that to 

 which the breed was originallj^ liable — as phthisis (con- 

 sumption) and dysentery. Thus, a stock of cattle pre- 

 viously subject to phthisis, sometimes become affected 

 for several generations with dysentery to the exclusion 

 of phthisis, but by and by, dysentery disappears to give 

 place to phthisis. 



4. Hereditary diseases occur to a certain extent inde- 

 pendently of external circumstances ; appearing under 

 all sorts of management, and being little affected by 

 changes of locality, separation from diseased stock, or 

 such causes as modify the production of non-hereditary 

 diseases. 



5. They are, however, most certainly and speedily 

 developed in circumstances inimical to general good 

 health, and often occur at certain, so called, critical 

 periods of life, when unusual demands on the vital 

 powers take place. 



■ R 



* Finlay Dun, V. S., in Journal of the Royal Agricultural Society. 



