164 EVOLUTION AND ANIMAL LIFE 



"Blood will tell," "Blood is thicker than water," these proverbs 

 in all languages indicate the general fact that each organism is 

 likely to resemble its parents, and that the basis of fundamental 

 resemblance among organisms is found in kinship by blood. It 

 is equally a matter of common observation that the law of hered- 

 ity is inseparable from a law of variation. No one organism is 

 quite an exact copy of another. The prevention of such a con- 

 dition is one of the effects of the process of double parentage. 

 Except in certain exceptional forms in which parthenogenesis 

 or hermaphroditism appear, each complex organism springs 

 from two organisms of the same species: the one male, the other 

 female. The resultant organism partakes of the qualities of 

 each of these in some degree, and through these to a degree also 

 it partakes of qualities of the parents or ancestors of each. 



The phrase, "Kinship by blood," used in connection with all 

 studies of heredity, is a survival of an ancient theory that the 

 physical basis of heredity is found in the actual blood. "Blood 

 is quite a peculiar juice," as was observed by Mephistopheles, 

 but its peculiarities are not concerned with heredity. The func- 

 tion of blood is concerned with the nourishment of tissues and 

 the removal of their waste. The actual vehicle of transfer of 

 hereditary qualities, the physical basis of heredity, is found in 

 structures within the protoplasm of the germ cell. 



The germ cells, male or female, are alike in all characters 

 essential to this discussion. On the average, the potency of the 

 male and the female cell is exactly the same, there being nowhere 

 constant advantage of one sex over the other. Each cell, male 

 or female, is one of the vital units, or body cells, set apart for 

 the special purpose of reproduction. It is not essentially differ- 

 ent from other cells in structure or in origin, but in its poten- 

 tialities. Its function is that of repeating the original organism, 

 ' with the precision of a work of art." 



Heredity is shown in the persistence of type, in the existence 

 of broad homologies among living forms, in the possibility of 

 natural systems of classification in any group, in the retention 

 of vestigial organs, in the early development and subsequent 

 obliteration of outworn structures once useful to individuals of 

 the race or type. 



In a general way, the individual inherits from both parents 

 the common structure of organisms of the species to which it 

 belongs. The special peculiarities of the individual organism 



