12 EVOLUTION AND DISEASE. 
types among normal cutaneous horns, not only in birds 
but among mammals, as parallels to the annual shedding 
of the pathological cutaneous horns of birds. 
Not infrequently tumours are found in certain abdo- 
minal organs and in the subcutaneous tissues of man 
and other mammals, possessing skin and its appendages 
such as hair, wool, and glands. Such tumours contain 
in man, horses, and oxen, hair; in pigs, bristles; in 
sheep, wool; and in birds, feathers ; thus harmonizing 
with the physiological characters special to the animal in 
which such tumours occur. Further, the hair in such 
tumour becomes grey as age advances, and like that on 
the exterior of the body is shed, so that such tumours 
may in the long run become literally bald. 
Without attempting to multiply instances, such facts 
as these were sufficient to induce me to pursue the 
inquiry into Zoological Pathology, or General Pathology 
in the fullest sense, and the latest views and investiga- 
tions in this wide, but little cultivated, field are sum- 
marized in the ensuing pages. 
Wherever possible, physiological types of diseased 
(pathological) processes are described ; the illustrations, 
whenever practicable, have been selected from animals 
other than man, for in him they have been too exclusively 
studied ; indeed, by restricting our inquiries to man it 
is impossible to frame any generalizations concerning 
disease upon a sound basis. It has been stated that “a 
knowledge of human anatomy is sufficient for the mere 
art of the surgeon.” This may be correct, but it is quite 
certain that if we restrict our observation of the processes 
of disease as they occur in man, our notion of them 
