No. 564] THE FIXATION OF CHARACTER 



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lies. A feature which is conservative and of diagnostic 

 value in one group may be variable and worthless in an- 

 other. The number of teeth and vertebrae, for example, 

 is much less constant among fishes than among mammals. 

 The general floral plan is far from uniform throughout 

 the Rosacea^, but in such families as the Cruciferae it is 

 exceedingly constant. This introduces still another prin- 

 ciple of conservatism which is really the crux of the whole 

 problem of fixation of character and seems to be a funda- 

 mental law of evolution— the principle that the progress- 

 ive evolution of any character or structure, whether in- 

 volving reduction or increased complexity, is attended by 

 a continual decrease in its tendency to change. Differenti- 

 ation and specialization are followed by increasing fixity. 



It is a well-known biological fact that the more primitive 

 families of animals and plants, those which still maintain 

 an ancient type of organization, are much more variable 

 in their characters than are those which have progressed 

 far from such a primitive condition. The lower Arthro- 

 poda, for example, display great variety in the number of 

 body segments and appendages and in many anatomical 

 features, but the highly specialized hexapod insects, de- 

 spite their enormous numbers, wide distribution and 

 extreme variation in size, shape and color, have become 

 rigidly stereotyped with regard to almost all characters of 

 number and general plan. In the ascending vertebrate 

 series from cyclostomes to mammals there are also many 

 instances of the increasing fixation of what we have called 

 conservative features, for it is well known that the char- 

 acters which make up the mammalian type are much more 

 definite and sharply circumscribed than those pertaining 

 to the lower groups of vertebrates where there is much 

 latitude in the distinguishing features. Likewise, the most 

 advanced and highly specialized families of plants, such 

 as the Composite and the Orchidaceo?, are characterized 

 by a stereotyped floral plan which is invariable throughout 

 all the members of these dominant groups, whereas in 

 plant orders admittedly lower in the scale, such as the 



