308 cook 



expended on this phase of the problem, but with no direct results. 

 For this reason, if for no other, the careful consideration of the 

 alternative possibility would be justified. 



The kinetic theory is not dependent, however, upon merely 

 abstract or inferential justification, but is supported by the evi- 

 dence of all observations and experiments which have a bear- 

 ing upon the question. That groups of organic individuals 

 become different whenever they have been isolated for any con- 

 siderable periods of time, may be taken as proof that evolutionary 

 change is a general and normal condition of the existence of 

 species. It can be asserted, of course, that divergences be- 

 tween groups of common origin are due to differences of 

 environment, but the inadequacy of this explanation is con- 

 clusively shown by the many instances where groups have pre- 

 served great similarity of habits and environmental conditions, 

 but have attained, nevertheless, to a great diversity of form and 

 structure, as in the conspicuous instance of the animals of the 

 class Diplopoda, and of various classes of the lower plants, such 

 as the mosses and hepaticae. 



Two modifications of the stationary assumption had been 

 formulated, previous to the kinetic theory. Under the muta- 

 tion theory of Professor De Vries, the normal condition of uni- 

 formity is supposed to give place at rare intervals to periods of 

 mutation or sudden appearance of new species. In the deter- 

 minant theory of Nageli, species were held to be normally in 

 motion, but the motion was supposed to follow a definite direc- 

 tion as the result of internal physical and chemical adjustments. 



The changes predicated as normal for species under the kinetic 

 theory are of an indeterminate and composite character. The 

 species is not thought of as changing in one direction merely, 

 but in many characters at once, the required result being a con- 

 structive coordination of changes which will increase the vita 

 efficiency of the organism and enlarge its power of utilizing its 

 environmental opportunities. 



RAPIDITY OF EVOLUTIONARY MOTION. 



Static theories, which have agreed in thinking of species as 

 normally stationary, have also taken it for granted that evolu- 



