756 COMPARATIVE PATHOLOGY. 



the progress of life. Thus, descending to particular cases, 

 we find that the long claws of the sloth and bat, the great 

 curved teeth of the Babirussa, are paralleled in pathological 

 conditions by the elongated nails and hoofs of Birds and 

 Ungulates kept in unnatural conditions, by the curved 

 incisors of Rodents which have lost the corresponding teeth 

 of the other jaw. It is unnecessary here to multiply 

 examples of greatly hypertrophied organs, normally present 

 in certain animals, but occurring in disease in others ; many 

 will suggest themselves. In considering many of these cases, 

 we must recognise the law of correlation, and realise that the 

 structures of a particular animal are not commonly the best 

 conceivable, but the best that can be attained under the 

 given conditions. 



Pathological new formations may arise in response to 

 mechanical stimulation, as in the case of corns and warts, or 

 may be due to aberrant physiological processes.. Thus 

 cancer is regarded by many as in origin an aberrant gland- 

 formation, and only occurs in regions of the body where 

 glands are normally found. It is a senile modification of 

 an ordinary developmental process. Pathological bony 

 growths seem to have their origin in patches of cartilage 

 remaining from the primitive cartilage of limb or brain-case, 

 and so are continuations of the ordinary process by which 

 cartilage is replaced by bone. 



Brief as the above comparative survey of Physiology and 

 Pathology is, it may serve to give the student some im- 

 pression of the intricacy of life, and act as a relief from 

 mechanical theories of Variation, Selection, and Heredity. 

 It is an attempt to look from the inner side upon the great 

 problem which is constantly being worked out before us, — 

 Given the potentialities of protoplasm and certain chemical 

 and physical conditions, to find the best adaptation to a 

 given environment. 



