708 THE AMERICAN NATURALIST [Vol. XLVII 



respective methods of reproduction ; but in a competition 

 between the two it is not primarily this difference which 

 decides the outcome. Differences in vegetative char- 

 acters, as well, and in the general vigor and adaptability 

 of the two plants determine which shall survive. One 

 of the most conservative and deeply-seated distinctions 

 between a mammal and a bird is the possession of hair 

 by one and of feathers by the other, but in the struggle 

 for existence between a bat and a night-hawk this differ- 

 ence is of very slight importance. The victor in such 

 competitions is that individual all of whose bodily parts 

 in their size, shape and general structure are so well 

 coordinated as to produce an organism with the greatest 

 degree of hardiness and adaptability. 



The conservative characters in each family or larger 

 groivp — its most important distinguishing features — pro- 

 vide a general plan of structure, a theme, on which are 

 produced the modifications of genera and species. It is 

 these modifications, involving the plastic and least con- 

 servative characters, which are of most importance in 

 adaptation and therefore in survival. The general plan 

 is of comparatively little significance in a contest — about 

 as much as is the particular make of modern rifle used 

 by an army or the special type of construction of a racing 

 car. A satisfactory interrelation and coordination of 

 parts is the important thing, and the degree of perfection 

 with which this is attained, on almost any plan, deter- 

 mines success or failure. It is true that after very long 

 periods of time in organic evolution slight differences 

 in value between two general plans of structure will some- 

 times make themselves felt and the best will finally be- 

 come dominant. Seed plants have little by little over- 

 come vascular cryptogams and mammals have superseded 

 reptiles. A highly adaptable organism, however, con- 

 structed on an " inferior" plan will often supplant one 

 which belongs to a generally superior type but is lacking 

 in versatility and vigor. The common bracken fern, for 

 example, a cryptogamous plant, is of almost universal 



