974 LIFE : OUTLINES OF GENERAL BIOLOGY 



extremely old type of horse, such as is hinted at in the striped 

 ponies of Tibet. 



There is no doubt that organisms often show peculiarities which 

 their parents did not possess, but which their ancestors possessed. 

 Summing up such cases descriptively , we may say that they seem 

 to illustrate atavism, but the use of the term as an interpretation 

 is not justified unless we can give some reasons for believing that 

 the resemblance to an ancestor is due to the rehabilitation of latent 

 items in the inheritance. To do this we have to try to eliminate 

 other interpretations, and that is often difficult, (a) What looks like 

 an ancient feature may be due to an arrest of development through 

 lack of appropriate nutrition, (h) Similar conditions of life, e.g. of 

 food and climate, may induce an acquired or modificational resem- 

 blance between the organism and its great-grandparent, but this 

 would not be an atavism, (c) Many organisms normally show certain 

 "vestigial organs", and these are often variable. A quantitative 

 variation in a normally present vestigial organ is not what is meant 

 by an atavism, (d) It is conceivable that an independent individual 

 variation may happen to coincide with one that occurred generations 

 before, but this is different from the reawakening of a latent item 

 in the inheritance, {e) Filial regression, or an approximation towards 

 the mean of the stock, is of everyday occurrence in blended inheri- 

 tance, and must be kept quite distinct from reversion or atavism. 

 (/) The list of alleged atavisms must also be reduced by the subtrac- 

 tion of weU-known Mendelian phenomena. In certain cases, such as 

 peas and mice, the crossing of two sharply contrasted pure-bred 

 parents results in hybrid offspring which are all like one of the two 

 parents as regards the contrasted characters; when these hybrid 

 offspring are inbred their progeny resemble in definite proportions 

 the two grandparents. 



The fact seems to be that many phenomena have been labelled 

 atavisms which admit of other interpretations, and that genuine 

 atavisms are rather rare. Let us repeat that an atavism is a harking 

 back to a more or less remote ancestor, the harking back being due 

 to the reassertion or reawakening of ancestral contributions which 

 have lain for several generations latent or unexpressed. 



It seems xmnecessary to use the term "atavism" for the common 

 phenomenon of resemblance to a grandparent. There is every reason 

 to believe that an individual inheritance is like a mosaic, built up 

 of many contributions, through the two parents, from the grand- 

 parents, great-grandparents, and so on. It is a normal and frequent 

 fact of inheritance that an offspring exhibits a peculiarity known 

 to have occurred in one of the grandparents but not in either of the 

 parents. There seems little utility in calling this very frequent 

 "skipping a generation" an atavism, though it is of the same general 

 nature, and though it is obviously difficult to decide where to draw 



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