XXII.] CLASSIFICATION. 281 



classes of the same great division, sucli as the Vertebrata. 



Thus far, I believe the development theory is as completely How far 



proved as any truth can be, of which we have neither lopment" 



direct evidence nor mathematical demonstration. More theory is 



proved, 

 than this I do not think we are yet in a position to assert 



with the same confidence ; but I believe that all organisms 



whatever are descended from a common origin. It will be Reason of 



seen why, on this view, so much importance in classifica- ™ge°of 



tion should be attached to organs that have become rudi- rudiment- 



mentary and aborted; for they show the descent of the 



organism as clearly as if they were conspicuous and at 



work. It will also be seen why the highest importance in 



classification is attached to those organs which are earliest 



formed in the embryo, and to embryonic characters gene- of embry- 



rally ; for the first developed characters, as shown in the racters ^" 



last chapter, are the least variable, and have probably 



varied the least throughout an indefinite number of 



generations. And it will be seen that although the of the 



flower and the seed-vessel in plants are not developed °^®^' 



early, yet they are of high classificatory importance for 



the same reason that embryonic characters are ; namely, 



that they are unlikely to be greatly changed by any change 



of an adaptive nature. Adaptation to a new habitat, for 



instance, or change of climate, will be much more likely 



to change its habit and mode of growth — to make the 



difference, for instance, between a tree and a herb — than 



to make any great change in the character of the 



flower. 



The foregoing remarks will all be familiar to the student 

 of Parwin. What follows is not absokitely original ; but 

 it has not yet, I think, been stated with the emphasis it 

 deserves. 



We suppose the following to be the rationale of the facts 

 of classification : — 



All living beings have a capacity for variation, which is 

 very limited in a single generation, but (like geological 

 change) quite unlimited if sufficient time is allowed, so 

 that the variations of successive generations may be added 



