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THE AMERICAN NATURALIST [Vol. XLVII 



corded ranges of variation and in so doing they have, as it 

 were, caught and fixed into the advancing and increas- 

 ingly specialized hexapod type the particular conservative 

 and functionless characters which happened to distinguish 

 those fortunate individuals which founded the present 

 family. As a result our modern hexapods, as a whole, 

 like all other natural orders, have as constant characters 

 certain peculiarities of number and plan, whereas the sub- 

 ordinate groups of the order are still distinguished, in 

 many cases, by the functionally important features to 

 which they owe their successful establishment, but which 

 in future evolution are doubtless destined to vary much. 



Similarly, in that ascending group of animals which 

 were to give rise to the higher vertebrates, the primitive 

 archipterygium became stereotyped into the pentadactyl 

 appendage, with its definite skeletal plan; but the par- 

 ticular improvements which caused the primitive penta- 

 dactylous stock to succeed at the start and to become 

 segregated as a new and distinct order were doubtless 

 concerned with such plastic but functionally important 

 characters as size and shape and with the general vitality 

 and adaptability of the race, and had little or nothing to 

 do with the particular number of digits or arrangement 

 of bones in the appendages. These characters, originally 

 variable, simply happened to belong to a successful and 

 progressive group of organisms and became fixed and 

 stereotyped as specialization took place. 



The ancestors of the grasses doubtless varied much as 

 to nodal structure, but the particular group which through 

 its success became the dominant and distinct modern 

 family happened to be characterized by the possession of 

 leaves whose bases formed an open sheath around the 

 stem and were provided with a small membranous struc- 

 ture, the ligule. These characters, which are doubtless not 

 the ones to which the family owes its success, since they 

 are present alike in dominant and in unsuccessful species, 

 became so firmly fixed during the progressive evolution 

 of the Gramineae that they now distinguish all members 



