l6o NATURAL HISTORY. 



time the glands by which they are enabled to suckle their 

 young. All animals possessing these two structures also 

 possess (or may possess) the special integumentary 

 appendages known as hairs. Similarly, all those animals 

 which have the stomach adapted for chewing the cud, or 

 ruminating, have as a correlation with this, no more than 

 two functionally useful toes, the third and fourth toes. 

 All such animals, moreover, have an incomplete develop- 

 ment of the incisor teeth in the upper jaw. They are also 

 the only living quadrupeds which have horns developed 

 upon the frontal bones. 



The few examples given above may suffice to illustrate 

 the general nature of the ' law of the correlation of organs.' 

 Stated in its most general form, this law asserts that all 

 the parts of the organism stand in some relation to each 

 other, the form and characters of each being in direct 

 connection with the form and character of all the rest. 

 The nature of this connection is in many cases hidden 

 from us ; but it is certain that if, by an arbitrary exercise 

 of will, we could suddenly change the form of any one 

 organ in any given animal, we should find ourselves com- 

 pelled to make changes in all the other organs of the same 

 animal. In many cases, perhaps, the changes necessitated 

 by the modification of some particular organ might be 

 slight ; in most cases we should be unable to see why any 

 changes should be needed at all, beyond the one with 

 which we had started ; in all cases the fact would remain, 

 that the living organism is an aggregate of parts so put 

 together that any modification of any one part necessitates 

 a modification of all the rest. 



The application of this law to palaeontology is easy to 



