CERTAIN LAWS OF VARIATION. 219 



to the relation between want of adaptation and varia- 

 bility in different organisms. Without doubt, how- 

 ever, these must vary enormously. 



It may be thought that this law is a mere reproduc- 

 tion, in different words, of one of the numerous causes 

 of variability discussed by Darwin. I do not think that 

 this is the case, however, for though he states that varia- 

 bility of every kind is due to changed conditions of life, 

 such as climate, food, and excess of nutriment, he does 

 not suggest that the real factor is want of adaptation 

 between the organisms and their surroundings. On 

 the contrary, he endeavours to show that the amount of 

 modification which animals and plants have undergone 

 does not correspond with the degree to which they have 

 been subjected to changed circumstances. 



From the conclusions at which we have arrived, it 

 follows that increased variability of environment leads 

 to increased variability of the organisms subjected to it. 

 A proof that this is the case has been afforded by Mont- 

 gomery,* who has studied the variation in migratory 

 and non-migratory species of North American birds. 

 From data collected by Eidgway in his " Manual of 

 North American Birds," he calculated the percentage 

 amount of variation (according as it was under 1 per 

 cent., between 1 and 1.5 per cent., between 1.5 and 2 

 per cent., or over 2 per cent.) in the culmen of the bill, 

 wing, tarsus, and tail of the species and sub-species of 

 fifty-six families. His results show " that migratory 

 species evince a greater amount of individual variation 

 than do non-migratory species ; and species which under- 

 take extensive migrations, a greater amount than 

 *J. Morph., 1896, p. 25, 



