THE STUDY OF HEREDITY 327 



To realise what is happening, it is necessary to appreciate 

 a certain property of living matter, a property which is abso- 

 lutely universal throughout the animal and vegetable kingdom 

 from amoeba to man, from algae and the like to the most highly 

 differentiated plants. This is the property of variation. No 

 two organisms or parts of organisms are ever exactly alike. 

 Living organisms consist of single cells or of groups of cells 

 living together. No two cells are ever exactly alike. When 

 I realise that every biologist believes in evolution of some 

 kind through some process of selection — and they all appear 

 to realise that variations in the offspring are necessary to evo- 

 lution — I marvel at the fact that so many theories exist to 

 account for the production of these variations during the later 

 stages of evolution. The variations must have been present 

 from the very first stage, otherwise evolution would obviously 

 have been impossible. A loss of the property of varying by 

 the cells forming any organism would of necessity have meant 

 that evolution and the appearance of new, and the increase or 

 diminution and disappearance of existing, characters would 

 have ceased. But actual observation shows that in no type of 

 cell has variation ceased. Examine the cells forming the most 

 highly differentiated tissues of the most highly differentiated 

 organism and you will never find two cells exactly alike. 

 This being the case, it must be perfectly obvious that the 

 organisms built up from these cells can never be exactly alike. 

 Offspring must always vary from their parents and offspring 

 of the same parents from each other. Sometimes the differences 

 are considerable, sometimes small. Obviously when minute 

 organisms with which the observer is not very familiar are 

 examined, these small differences will escape his notice. 

 Familiarity is a great factor. To the white man all negroes 

 appear alike, but when he has lived among them for some years 

 he sees as much difference between them as between his 

 fellow white men. In the case of microscopic animals and 

 plants, small differences are even more likely to escape notice, 

 but a careful examination by a skilled observer shows that 

 they are always there. 1 Naturally if the environment of an 

 organism remain unchanged for a long period of time, any 

 variations which tend to interfere with adaptation will be 



1 I have dealt with many cases in which variation has been claimed as absent 

 in Hereditary Characters (Arnold, London, 19 10). 



