CHAPTER IV 



Variation De Vries' mutation theory The case of CEnothera The disappear- 

 ance of characters Variation in grafts The cell layers in the embryo 

 Variations in wild and domesticated races Arguments against the origin 

 of species per saltum Fertility of hybrids. 



THE similarity of offspring to their parents is very marked 

 in the case of both animals and vegetables, and extends to 

 all the characters of the race. It is, however, a matter of 

 common knowledge that among the higher animals at any 

 rate, offspring differ both from their parents and from each 

 other to some extent. Variability appears to be a property 

 common to all living organisms in spite of the fact that 

 individual animals and plants produce new individuals that 

 are generally similar to themselves. For instance, if a collie 

 dog be mated with another collie, the pups produced will 

 grow to about the same size as their parents ; they will have 

 similarly shaped heads, and be similar generally. Every 

 individual, however, in the litter of pups, will differ in some 

 way from its brothers and sisters, and also from its parents. 

 But though these differences are very evident, upon careful 

 examination they are in the overwhelming majority of 

 instances comparatively small differences ; the pups will, 

 in fact, though differing from their parents, still resemble 

 them beyond all comparison more nearly than they will 

 resemble a fox-terrier or any other breed of dog. Although 

 among the lower animals and plants this appearance of 

 difference between parent and offspring, and between 

 different offspring of the same parents, is not nearly so 

 noticeable, owing to our scanty powers of observation due 

 to lack of familiarity with the organisms concerned, still this 

 variation is of universal occurrence throughout the animal 

 and vegetable kingdoms. Before considering the theories 



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