654 
EDTTORTAL OBSERVATIONS. 
on our part, so much the more discriminative judgment in 
detecting the genuine or original type, or essential character 
of the breed. 
In regard to the hereditary transmission of disease, grant- 
ing the fact as demonstrable beyond question, in the animal 
as well as in the human race, Mr. Dun shows that such 
transmission operates under certain defined influences, 
amounting, so far as they have been traced, to the follow- 
ing recognised laws : 
“ 1. They (hereditary diseases) are transmitted by the male 
as well as by the female parent, and are doubly severe in 
the offspring of parents, both of which have been affected by 
them. 
2. They develop themselves, not only in the immediate 
progeny of animals affected by them, but also in many 
subsequent generations. 
3. They do not, however, always appear in each genera- 
tion exactly in the same form. One disease is sometimes 
substituted for another analogous to it, and this, after some 
generations, becomes again changed into that to which the 
breed was originally liable. Thus, stocks of cattle previously 
subject to phthisis, often become affected for several genera- 
tions with dysentery, to the total exclusion of phthisis ; 
but, by and by, the dysentery disappears to give place to the 
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 the diseased stock, or such other 
causes as modify the production of non-hereditary dis- 
eases. 
5. They are, however, more speedily and certainly developed 
in circumstances inimical to general good health, and often 
occur at certain so-called critical periods of life, when un- 
usual demands on the vital powers take place. 
6. They show a striking tendency to modify and absorb 
into themselves all extraneous diseases. For example, in an 
animal of a consumptive constitution, pneumonia seldom 
