PRESIDZXTIAL ADDRESS. 257 



bility of every kind is directly or indirectly due to changed 

 conditions of life. Are the effects of such changed con- 

 ditions determinate or indeterminate? There can be no 

 question that in many cases the changes are quite definite. 

 Changed food or changed climate produce particular and 

 almost startling alterations. Instance the case of the Texan 

 Saturnia^ which when fed in Switzerland on a new plant 

 {JiigJans regia instead of J. nigra) was so altered both 

 in colour and form that it appeared to be a new species. 

 Instance again the remarkable changes in brine-shrimps 

 produced by changes in the salinity of the water. Few 

 biologists will be likely to question the determinate effects 

 of the action of the environment on the individual. 



On the whole, having lately had occasion to review the 

 evidence with regard to the nature of variations, I am 

 drawn toward the following conclusions : (1) that correlated 

 variations tend to occur in definite or determinate lines ; 

 (2) that under natural selection some of these determinate 

 variations have presumably been eliminated ; but (3) that 

 there is no sufficient evidential justification of the assertion 

 that the determinate lines of variation are solely the pro- 

 ducts of natural selection. 



"We come now to the question of origin. 



Undoubtedly an important source of origin of the varia- 

 tions which occur in the higher animals and plants is 

 parental commixture. When two parents each contribute 

 something towards the formation of offspring, there are four 

 logical possibilities : (1) Characters may be inherited exclu- 

 sively from one parent or the other. If all the characters 

 are inherited from one parent, the offspring will resemble 

 that parent. If some characters are inherited exclusively 

 from one parent, and other characters from the other parent, 

 the offspring will present a new combination of the parental 



