92 Tin- Unit i) of ilic 



accepted without question as applicable in all the cases, is 

 a guarantee that it has more in common in all the cases 

 than the attributes by which it is familiarly recognized. For 

 instance the redness of the fluid in all vertebrates "makes it 

 almost certain that with the obviously common color there 

 will be other common attributes not obvious to ordinary ob- 

 servation. And so it happens that our physiologist goes 

 to the dog or the ox or the horse or the rabbit, or with a 

 little more hesitancy, to the frog for his supply, these being 

 as a rule the most convenient sources. 



One cannot reflect too carefully on the difference between 

 this mode of approach to the phenomena of organisms, and 

 that peculiar to the natural historian. Consider a moment 

 the difference as touching one thing, the blood, and as be- 

 tween only two animals, the dog and ox let us say. Notice 

 first that whereas the physiologist's primary interest would 

 be satisfied with what he might get from a study of the blood 

 of either of these animals without reference to the other, 

 not so with the naturalist. Just blood is what the physiolo- 

 gist is concerned with in this particular scries of researches. 

 Comparison is no essential part of his enterprise. All he 

 does in this way is incidental, is secondary. Could he get all 

 the dog's bliood he needed to make out all that can be learned 

 about that blood, he would never concern himself as a strict 

 physiologist about the blood of any other animal. The 

 physiologist so far as he holds himself to his calling, and 

 accepts his calling as it has delimited itself in modern times, 

 namely, as having for its field oue aspect of organisms, 

 i.e., their functions, can never consistently go outside of 

 function except incidentally except for such morphological 

 facts as are essentially related in one way or another to 

 function. To the physiologist blood is blood no matter 

 whether from a dog or an ox. 



To the zoologist, on the other hand, blood is never /just 

 blood. It is always blood of a dog or of an ox, or of some 



