152 Development of the Natural System under [BOOK i. 



and methodical thinker like Darwin 'was able to draw from 

 them the chief supports of the theory of descent. For it is 

 quite certain that Darwin has not framed his theory in opposi- 

 tion to morphology and system, and drawn it from any hitherto 

 unknown principles ; on the contrary, he has deduced his most 

 important and most incontestable propositions directly from 

 the facts of morphology and of the natural system, as it had 

 been developed up to his time. He is always pointing ex- 

 pressly to the fact that the natural system in the form in which 

 it has come to him, which he accepts in the main as the true 

 one, is not built upon the physiological, but upon the morpho- 

 logical value of organs ; it may, he says, be laid down as 

 a rule, that the less any portion of the organisation is bound 

 up with special habits of life, the more important it is for 

 classification. Like Robert Brown and De Candolle, he insists 

 upon the high importance for purposes of classification of 

 aborted and physiologically useless organs ; he points to cases 

 in which very distant affinities are brought to light by numerous 

 transition-forms or intermediate stages, of which the class of 

 the Crustaceae offers a specially striking example in the animal 

 kingdom, while certain series of forms of Thallophytes, the 

 Muscineae, the Aroideae and others, may be adduced as in- 

 stances of the same kind in the vegetable world ; in such 

 cases the most distant members of a series of affinities have 

 sometimes no one common mark, which they do not share 

 with all other plants of a much larger division. From these 

 and other similar statements of Darwin we see plainly, that he 

 actually did gather from existing natural systems of plants and 

 animals the rules by which systematists had worked, but which 

 they themselves observed only more or less unconsciously, and 

 never with a full and clear recognition of them. He says quite 

 rightly, when the investigators of nature are practically engaged 

 with their task, they do not trouble themselves about the 

 physiological value of the characters which they employ for 

 the limiting a group or the establishment of a single species. 



