IMPORTANCE OF AFFINITIES 31 



Affinities are always incomplete when they apply only in an isolated 

 case ; that is to say, when they are decided from an examination 

 of a single part taken by itself. But, although incomplete, the value 

 of affinities based upon a single part depends upon the extent to which 

 the part from which they are taken is essential, and vice versa. 



There are then determinable differences among af&nities, and various 

 degrees of importance among the parts which display them ; ia 

 fact, the knowledge of af&nities would have had no apphcation or 

 utihty unless the more important parts of living bodies had been 

 distinguished from the less important, and unless a principle had been 

 found for estimating the true values of these important parts. 



The most important parts for exhibiting the chief affinities are, 

 among animals, the parts essential to the maintenance of life, and 

 among plants, the parts essential to reproduction. 



In animals, therefore, it is always the internal organisation that will 

 guide us in deciding the chief affinities. And in plants, it will be 

 in the parts of fructification that affinities will be sought. 



But in both cases the parts most important for seeking out affinities 

 vary. The only principle to be used for determining the importance 

 of any part, without arbitrary assumptions, consists in enquiring 

 either how much use nature makes of it, or else the importance to the 

 animal of the function of that part. 



Among animals, whose affinities are mainly determined by their 

 internal organisation, three kinds of special organs have rightly been 

 chosen from among the others as the most suitable for disclosing 

 the most important affinities. They are, in order of importance, as 

 follows : 



(1) The organ of feeling. The nerves which meet at a centre, either 

 single as in animals with a brain, or multiple as in those with a 

 gangHonic longitudinal cord. 



(2) The organ of respiration. The lungs, gills and tracheae. 



(3) The organ of circulation. The arteries and veins, which usually 

 have a centre of action in the heart. 



The first two of these organs are more widely used by nature, and 

 therefore more important than the third, that is to say, the organ of 

 circulation ; for the latter disappears in the series after the crustaceans, 

 while the two former extend to animals of the two classes which follow 

 the crustaceans. 



Finally, of the two first, the organ of feeling has the more import- 

 ance from the point of view of afSnities, for it has produced the most 

 exalted of animal faculties, and moreover without that organ muscular 

 activity could not take place. 



If I were to refer to plants, among which the reproductive parts 



