30 ZOOLOGICAL PHILOSOPHY 



The question of affinities does not apply only to species ; for we 

 have also to fix the general affinities of all the orders into which groups 

 are united or divided from the comparative point of view. 



Affinities, although possessing very different values according to the 

 importance of the parts exhibiting them, can none the less be extended 

 to the conformation of the external parts. If the affinities are so great 

 that not only the essential parts, but also the external parts present 

 no determinable difference, then the objects in question are only 

 individuals of the same species. If on the other hand, notwith- 

 standing a large degree of affinity, the external parts exhibit appreci- 

 able differences, though less than the essential resemblances ; then 

 the objects in question are different species of the same genus. 



The important study of affinities is not limited to a comparison 

 of classes, families, or even of species ; it includes also a considera- 

 tion of the parts of which individuals are composed. By comparison 

 together of corresponding parts we obtain a firm basis for recognis- 

 ing either the identity of individuals of the same race or the difference 

 between individuals of distinct races. 



It has, in truth, been noted that the proportions and relations of 

 the parts of all individuals composing a species or a race always 

 remain the same, and so appear to be preserved forever. From this 

 it has been rightly inferred that, by examining detached parts of an 

 individual, one could decide to what species, old or new, these parts 

 belong. 



This power is very favourable to the progress of knowledge at the 

 present time. But the conclusions drawn from it can only hold good 

 for a limited period ; since the races themselves undergo changes 

 in their parts, in proportion to any considerable change in the circum- 

 stances which affect them. As a matter of fact, since these changes 

 only take place with an extreme slowness, which makes them always 

 imperceptible, the proportions and relations of the parts always appear 

 the same to the observer, who does not really see them change. 

 Hence, when he comes across any species which have undergone 

 these changes, he imagines that the differences which he perceives 

 have always existed. 



It is none the less quite true that by a comparison of corresponding 

 parts in different individuals, their affinities, nearer or more remote, 

 can be easily and certainly determined. It can therefore be known 

 whether these parts belong to individuals of the same race or of 

 different races. 



It is only the general inference that is unsound, having been drawn 

 too hastily. This I shall have more than one opportunity of proving 

 in the course of the present work. 



