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THE AMERICAN NATURALIST [Vol. XLVH 



struggle for existence, which have consequently been 

 firmly standardized and kept rigidly true to type by the 

 action of natural selection in continually eliminating those 

 individuals which showed a tendency to depart from the 

 normal condition. The invariable presence of segments 

 in the Articulata, of trachea? in insects, of the backbone 

 in vertebrates, of gills in fishes, of feathers in birds, of 

 roots in the vascular plants, of seeds in the spermato- 

 pliytes and of vessels in the wood of the angiosperms, all 

 of which are characters of universal occurrence in the 

 groups which they distinguish, is explained as due to 

 their supreme importance for survival. The frequent 

 variability of rudimentary or obviously useless struc- 

 tures is laid to their unimportance in the struggle 

 for existence and their consequent removal from the 

 standardizing influence of natural selection. This belief 

 in the dependence of structural conservatism on func- 

 tional utility is widely held to-day and has been stated 

 by Montgomery as follows: "A character which per- 

 sists through a very long racial period must do so by 

 virtue of being of particular value for the economy of the 

 organization or for the perpetuation of the race. Struc- 

 tures of less value are more readily modified, substituted 



A strict application of the selection hypothesis, how- 

 ever, evidently fails to explain many facts which a study 

 of phylogeny brings forward. Can we imagine, for ex- 

 ample, that either the number five, on which echinoderms 

 are built, or the number three, wliidi i> characteristic of all 



the struggle for existence? Is it logical to suppose that 

 the position of the protoxylem with reference to the later- 

 formed elements of the vascular axis, a position which is 

 extremely constant throughout the main groups of vas- 

 cular plants, has been definitely determined by natural 

 selection, or that the precise number of floral parts or the 



2 Montgomery, T. H., "On Phylogenetic Classification, » ' Troc. Philadel- 

 phia Acad. Sri., Vol. 54, 1902, p. 214. 



