52 Hypothetical Bearers of Specific Characters 



This makes him assume that the units of different indi- 

 viduals are slightly dissimilar. From this it follows that 

 in the child the two kinds of units of both parents are 

 mixed, in the grandchild the four different units of the 

 grandparents, and so on. In this way one would arrive 

 at just the opposite of what was at first assumed, namely, 

 the similarity of all units in the same individual (pp. 253, 

 254, and 267). 



To escape this difficulty Spencer points to hybrids. In 

 these the physiological units of two species are mixed. 

 The hybrids are liable to be inconstant in the following 

 generations, and to revert to the parental forms. There- 

 fore the unlike physiological units oppose a mixture, they 

 repulse each other, and try each, by excluding the dis- 

 similar kind, to form the whole individual (p. 268). In 

 the same manner the unlike physiological units exclude 

 each other in normal fertilization, and in this way uni- 

 formity within the individual is sufficiently assured. 



The physiological units multiply at the expense of the 

 nutrient material (p. 254) and thus produce, as a rule, 

 new units that are quite alike. Under the influence of 

 external circumstances, however, they sometimes undergo 

 slight changes during the process of their multiplication, 

 and this is the cause' of their variability (p. 287). 

 Through fertilization, however, the balance thus disturbed 

 is regained (p. 289). 



On this basis heredity is easily explained ; it is founded 

 on the fact that the child receives from father and mother 

 the material units that go to make up its characters. 

 Strong resemblance of the child to one of its two parents 

 is due to the predominance of the respective physiological 

 units; atavism depends upon the presence of units in- 

 herited from some given ancestor. Many other phenom- 



