transmitted to the offspring. His general conclusion was 

 that heritable variations are due to external conditions act- 

 ing directly on the germ-plasm, and that one of the most 

 important external conditions has been the food supply 

 {Variation af Plants and Animals). 



Variations have very appropriately been termed" the 

 raw materials of evolution," for if there were no variations 

 there could be no evolution. The production of the highly- 

 developed domesticated breeds of animals and plants of 

 to-day is the result of man's conscious selection of those 

 forms that showed variations in directions that suited his 

 fancy. 



Darwin supposed that organisms under domestication 

 tend to vary far more than similar organisms do in the wild 

 state, a result due, he thought, to more varied surroundings. 

 Later investigators like Bateson inform us, however, that 

 variation is as common among wild animals as it is among 

 domesticated. 



During the last forty years important progress has 

 been made in the study of variations. Quetelet, a Belgian 

 investigator, showed that ordinary variability follows the 

 law of probability, and this conclusion has been frequently 

 verified by later investigators. Whenever a large number 

 of organisms of a kind are compared, and similar parts are 

 carefully measured or counted, it is seen that the variations 

 in any single character are very numerous. The larger 

 variations from the average type in either direction are 

 fewer than the smaller variations, and if these variations 

 are plotted in paper, according to their size and frequency, 

 a curve of frequency is obtained which corresponds to the 

 law of probabilities. This curve has a crest decreasing 

 rapidly to zero on either side. 



According to this curve variability is limited and tends 

 to return to the average condition. (See page 25). 



Variations that behave in this manner are termed 

 fluctuating, variations, since they fluctuate about their 

 average or mean. 



The other kind of variations, viz., discontinuous varia- 

 tions,^ has been shown by Bateson^ (Materials for 



(1) — Galton many years before illustrated the difference between con- 

 tinuous and discontinuous variations by the polygon. 



(2) — Prof W. Bateson, the noted English biologist and geneticist, was 

 born in 1861 and educated at Rugby and Cambridge. He has 

 filled the professorship of Biology at Cambridge, and of Physi- 

 ology of the Royal Institution, and is now Director of the John 

 Innes Horticultural Institution, Merton Park. Surrey. He was 

 awarded the Darwin Medal in 1904, and was President of the 

 BA4S 1914. ■ His publications are— "Materials for the Study 

 of Variation' (1894), "Mendels Principles of Heredity" (1902) and 

 "Problems of Genetics." 



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