340 CLASSIFICATION. [CHAP. XIV. 



or by the species having become extinct in the intermediate tracts. 

 Both in time and space species and groups of species have their 

 points of maximum development. Groups of species, living during 

 the same period of time, or living within the same area, are often 

 characterised by trifling features in common, as of sculpture or 

 colour. In looking to the long succession of past ages, as in looking 

 to distant provinces throughout the world, we find that species 

 in certain classes differ little from each other, whilst those in 

 another class, or only in a different section of the same order, differ 

 greatly from each other. In both time and space the lowly organised 

 members of each class generally change less than the highly 

 organised ; but there are in both cases marked exceptions to the 

 rule. According to our theory, these several relations throughout 

 time and space are intelligible ; for whether we look to the allied 

 fonns of life which have changed during successive ages, or to 

 those which have changed after having migrated into distant 

 quarters, in both cases they are connected by the same bond of 

 ordinary generation ; in both cases the laws of variation have been 

 the same, and modifications have been accumulated by the same 

 means of natural selection. 



CHAPTER XIV. 



MUTUAL AFFINITIES OF ORGANIC BEINGS : MORPHOLOGY : 

 EMBRYOLOGY : RUDIMENTARY ORGANS. 



CLASSIFICATION, groups subordinate to groups Natural system Rules and 

 difficulties in classification, explained on the theory of descent with modifi- 

 cation Classification of varieties Descent always used in classification 

 Analogical or adaptive characters Affinities, general, complex, and 

 radiating Extinction separates and defines groups MORPHOLOGY, be- 

 tween members of the same class, between parts of the same individual 

 EMBRYOLOGY, laws of, explained by variations not supervening at an early 

 age, and being inherited at a corresponding age RUDIMENTARY ORGANS"; 

 their origin explained Summary. 



Classification. 



FROM the most remote period in the history of the world organic 

 beings have been found to resemble each other in descending degrees, 

 so that they can be classed in groups under groups. This classi- 

 fication is not arbitrary like the grouping of the stars in constel- 

 lations. The existence of groups would have been of simple 

 significance, if one group had been exclusively fitted to inhabit 

 the land, and another the water; one to feed on flesh, another on 

 vegetable matter, and so on ; but the case is widely different, for 



