SUMMARY AND CONCLUSION 463 



of modifying the determinants of the germ-plasm. Climatic 

 influences, for example, may very well produce permanent 

 variations, by slowly causing gradually increasing alterations 

 to occur in certain determinants in the course of generations. 

 An apparent transmission of somatogenic modifications may 

 even take place under certain circumstances, by the climatic 

 influence atfecting certain determinants of the germ-plasm at 

 the same time, and wlien they are about to pass to that part of 

 the body which they have to control. This is indicated by the 

 climatic variations of the butterfly Polyonuuatiis phlcras. 



The primary causes of variation is always the effect of 

 external influences. Were it possible for growth to take 

 place under absolutely constant external influences, variation 

 would not occur; but as this is impossible, all growth is con- 

 nected with smaller or greater deviations from the inherited 

 developmental tendency. 



When these deviations only aiTect the soma, they give rise to 

 temporary non-hereditary variations ; but when they occur in 

 the germ-plasm, they are transmitted to the next generation and 

 cause corresponding hereditary "iiariations in the body. 



Since the germ-plasm undergoes a very considerable growth 

 from the fertilised egg-cell to the germ-cells of the off"spring, 

 minute fluctuations continually take place in the composition 

 of its vital units, the biophors and determinants. If permanent 

 and constant influences, such as those of climate, act upon them. 

 these minute fluctuations will become accumulated in the course' 

 of time and generations, and may thus give rise to appreciable 

 individual variations, and then gradually to racial, and even 

 perhaps to specific characters. If an influence acts in a certain 

 direction for a short time only, it alone may or may not give 

 rise to an individual variation in the soma, according to the 

 number of ids of the germ-plasm aftected by it. Whenever 

 a majority of ids become modified, a corresponding variation 

 must appear in the soma. As, however, an intermingling of 

 the ids takes place twice, owing to the successive proces.ses 

 of ' reducing division ' and amphimixis, minorities of modified 

 ids may be increased to majorities ; and sexual reproduction 

 may then cause the fluctuating material for invisible variations 

 in the determinants to give rise to perceptible somatic variations, 

 and these are made use of by natural selection, aided by con- 

 stantly recurring amphimixis. The latter process gives natural 



