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THE AMERICAN NATURALIST [Vol. XLVII 



The character of general plan, or type, which really in- 

 cludes those of number and position, is of the utmost 

 importance for the discovery of relationships. In every 

 natural group of organisms, no matter how dissimilar its 

 members may appear, there is always a specific plan or 

 theme which is common to all and upon which the struc- 

 ture of each individual is built. The two-layered or three- 

 layered body plan, the presence or absence of segmenta- 

 tion, the definite type of arthropod or vertebrate append- 

 age which is so constant throughout its endless modifica- 

 tions, the plan of the central nervous system in the verte- 

 brates, and the precise and unvarying character of the 

 epidermal structures in the different classes of that 

 phylum, are a few of the innumerable examples of the con- 

 servatism of type in the animal kingdom. In the case of 

 plants the same fact is no less evident. The general topog- 

 raphy of the vascular system, the presence or absence of 

 leaf-gaps, the degree of differentiation in the structure of 

 the wood and the open or closed character of the leaf vena- 

 tion are all extremely constant. The notable conservatism 

 of type in the reproductive organs of all plants is well 



almost complete uniformity throughout animals and plants 

 of many cytological characters, such as those concerned 

 with mitosis, might also be cited as striking examples of 

 the conservatism of plan or type. 



Plastic and variable characters, no less than conserva- 

 tive ones, are separable into categories, the most im- 

 portant of which are size, shape, color and texture, of 

 which the inconstancy is so notorious that any broad 

 classification based upon them is very rarely a natural one. 



But even if we admit that certain characters are essen- 

 tially more slow to change than others, it is very evident 

 that this difference is not an absolute one, but that "con- 

 servative" features may display a greater or a less de- 

 gree of constancy in certain parts of the organism than 

 in others. These differences in local variability, however, 

 like those between general categories of characters, are 



