DARWIN AND THE ORIGIN OF SPECIES 255 



arrangement of the strongest parts, first a girder, next an arch, 

 and finally a suspension bridge. Yet the outline might remain 

 the same, and not a single member be removed. 



Thus we see, that though in three distinct and extreme cases 

 it is easy to give distinctive names with clear characteristics, it 

 is very difficult as the varieties multiply to draw distinct lines 

 between them. Shall the distribution of strains be the impor- 

 tant point ? Then one and the same piece of framework will 

 have to be included under each of three heads, according to the 

 manner in which it is suspended or supported. Shall form be 

 the important point ? We may construct a ribbed arch of 

 string, of a form exactly similar to many compressed arches, 

 we may support this from below, and yet the whole arch shall 

 be in tension, and bear a considerable load. Shall the mode of 

 support be the important point ? It would be an odd conclu- 

 sion to arrive at, that any stiff beam hung up in a particular 

 way was a suspension bridge. Nor is this difficulty simply a 

 sophistical one invented for the occasion ; the illustration was 

 suggested by a practical difficulty met with in drawing up a 

 patent ; and in ordinary engineering practice one man will call 

 a certain bridge a stiffened arch, while another calls it a girder 

 of peculiar form ; a third man calls a bridge a strengthened 

 girder, which a fourth says differs in no practical way from a 

 suspension bridge. Here, as in the case of animals or vege- 

 tables, when the varieties are few, classification is comparatively 

 easy ; as they are multiplied it becomes difficult ; and when all 

 the conceivable combinations are inserted it becomes impos- 

 sible. Nor must it be supposed that this is due to the sugges- 

 tion of one form by another in a way somewhat analogous to 

 descent by animal reproduction. The facts would be the same 

 however the bridges were designed. There are only certain 

 ways in which a stream can be bridged ; the extreme cases are 

 easily perceived, and ingenuity can then only fill in an indefi- 

 nite number of intermediate varieties. The possible varieties 

 are not created by man, they are found out, laid bare. Which 

 are laid bare will frequently depend on suggestion or associa- 

 tion of ideas, so that groups of closely analogous forms are dis- 

 covered about the same time ; but we may a priori assert that 

 whatever is discovered will lie between the known extremes 



