THE VITAL FAIJRIC OK DESCENT 3O9 



very rare and local species are correctly looked upon as rem- 

 nants verging toward extinction rather than as ascendant new- 

 born types. 



Some have thought to reconcile the idea of a progressive 

 evolution with the older notion of constancy of characters 

 among the members of a species by supposing that evolution- 

 ary changes proceed by imperceptibly gradual, infinitesimal 

 steps, and must therefore have required millions on millions of 

 years. As a matter of fact, however, differences between the 

 individual members of species in nature are commonly quite 

 perceptible, and often strikingly obvious. 



It has been attempted, also, to distinguish between what are 

 called continuous, or gradual, and discontinuous, or saltatory, 

 variations, the former to be found within specific lines, the 

 latter initiating new species. This distinction is artificial and 

 misleading ; variations may be discontinuous but they do not 

 disconnect the species. No reason is apparent why a species 

 might not be completely transformed within a few years, dec- 

 ades or centuries through the acceptance, by all of its mem- 

 bers, of a new character or characters. Instances where such 

 changes appear to be going on have been adduced by several 

 naturalists. Evolutionary progress can be accomplished in this 

 way much more rapidly than if it were necessary to replace the 

 older form of the species with the progeny of a mutation, which 

 needs to be kept isolated from the older species lest it be swamped 

 by intercrossing. Prepotency, the power to transform the 

 species, instead of being swamped, is the practical difference 

 between genetic variations and mutations.^ 



The kinetic theory sets no limits to the length of the steps, 

 nor to the rapidity with which they may be taken. It implies, 

 however, that the evolutionary progress of the species goes for- 

 ward as a network of descent, broken neither by sudden trans- 

 formations nor by periods of stationary constancy. As far as 

 our present perceptions carry us, variations may appear fortui- 

 tous. Evolution, however, is not accidental nor casual, but 

 necessary and universal. Neither is it passive nor intermittent, 

 but persistently and continuously conservative and constructive. 



^The Evolutionary Significance of Species, Smithsonian Report for 1904, p. 

 397- 



