XX Introduction. 



then found to be correlated with the complete or nearly complete 

 disappearance of such structures, must be regarded as entailing- 

 a true morphological degradation. 



Sharply circumscribed outlines are, in the second place, as com- 

 monly wanting in the classifications we have to deal with as are 

 precisely graduated scales of dignity. The boundaries of species, of 

 orders, of classes, and, in more than one instance, even of sub-king- 

 doms, may be closely apposed not only at many single points 

 but even along considerable lengths and depths, so that in not a 

 few cases it is a matter of difficulty to decide whether a particular 

 organism or set of organisms shall be placed within the one or the 

 other of the thus complexly approximated groups. 



The distances, thirdly, which intervene between the various sub- 

 kingdoms at their points of widest separation from each other are 

 exceedingly unequal. And in particular, it may be said that, in 

 spite of recent discoveries'', it would still appear that the Vertebrate 

 Sub-kingdom lies at a greater distance from the group made up by 

 all the other Sub-kingdoms than that by which any one of these is 

 separated from its nearest neighbour. 



Groups, fourthly, which would be allowed on all hands to possess 

 the same morphological or qualitative rank are found to differ very 

 widely as to the numbers of the objects they severally include ; 

 and if, by the aid of diagrams, we represent to ourselves the quan- 

 titative relations which the corresponding divisions in almost any 

 two of the animal sub-kingdoms hold to each other as wholes of 

 * extension' or of ' denotation,' we are at once struck by the great 

 inequality of size indicated by the figures thus constituted. A 

 similar result would ensue upon the application of a similar process 

 to many non-biological classifications ; the especial significance 

 which these differences possess in organic classifications depends 

 upon two singular but suggestive facts, actual observation having 

 shown, firstly, that the poorer a species is numerically the more 

 aberrant is it ordinarily found to be from the type of the group 

 to which it is subordinated ; and secondly, that with a paucity 

 of individuals or of species, as the case may be, a similar paucity 

 of the localities on the earth's surface in which they are now to be 



b For a suramary of these discoveries, see the Quarterly Journal of Microscopical 

 Science, January, 1870. 



