VARIATION AND HEREDITY 211 



Professor Pearson's studies upon the poppy, in which 

 he found a. very marked correlation between the 

 fertility of the plant, as indicated by the seeds- 

 developed, and the number of stigmatic bands on 

 the seed capsule. 



Effect of Life Conditions upon Variation. It has 

 been shown, both by comparative studies on plants 

 and animals at different ages, and also, experi- 

 mentally, by varying the external conditions, that 

 the younger stages of ontogeny show a much greater 

 range of variability ; in other words, that there is a 

 progressive reduction of variability in development. 

 It must be kept in mind constantly that general 

 statements of this sort apply to masses of individuals 

 and not to single individuals. The weeding out of 

 the extreme variates in the course of development 

 would bring about a similar result. 



Suddenly and profoundly altering the conditions 

 of life appears sometimes to increase the variability 

 of organisms* The English sparrow was imported 

 into America about the middle of the last century, 

 and its new surroundings proved so favorable that 

 it soon spread over the whole country. The eggs 

 of 868 sparrows from America and an equal number 

 from England were studied, and it was found that 

 the former were considerably more variable both in 

 size and in color. Whether one is justified in extend- 

 ing such a conclusion into a general law may be 

 questioned. 



Thus it has been long thought that domestication 



