Nos. 622-623] MIGRATION A FACTOR IN EVOLUTION 473 



He then quotes Vernon and continues : 



If the pressure on a liquid is made less than the vapor pressure for 

 that liquid at that temperature, some of the liquid vaporize. II e tem- 



ditions. What would happen if the liquid were not adaptable? The 



under it. The temperature rises until the boiling point is reached. The 

 liquid then ceases to be adaptable. It volatilizes, it disappears, it bo- 

 new point of view to conside/the extinction of the inasio,!,.,, a> anal- 

 ogous to the distillation of water; but the two cases are really parallel 



These facts are of the greatest importance because 

 they indicate the critical stcif/c or condition at which, in 

 the migration of animals into new localities and condi- 

 tions, organisms are most likely to be modified, and thus 

 influence their evolution. This furnishes a new reason 

 for stressing the importance of the breeding conditions 

 and habitat in ecology. 



An exception to Vernon's law is to be seen in the case 

 of the Protozoans (and probably to other kinds of non- 

 sexual reproduction), as is indicated by Jennings ('12, 

 '13) and Woodruff's investigations, which show that in 

 a proper environment the inertia of the life cycle tends to 

 continue on indefinitely and does not run down. J ennings 

 ('12, pp. 573-574) shows that conjugation in Protozoa 

 and sexual reproduction in the metazoa cause diverse and 

 new combinations of characters. In other w T ords, this 

 means that the processes of conjugation (favored by ad- 

 verse conditions ) and sexual reproduction, tends to break 

 up the stability and crystallization into which the on- 

 togenetic system tends to develop, and tends to restore, 

 as it were, flexibility and a colloidal state to the race. 

 This changes the system so as to minimize the interfer- 

 ence with its processes. In the metazoan the number of 

 systems is so large, that in spite of its chemical integra- 

 tion and regulation, interference with one or more of 



