FOUR IMPORTANT THEOREMS 53 



being better qualified than any equally well-placed competitor 

 to, gain a lodgment. Thus the step-by-step development of the 

 embryo cannot fail to be influenced by an incalculable number 

 of small and mostly unknown circumstances." [Natural Inherit- 

 ance, p. 9.) 



3. Duality of Inheritance may be real, though it is not ex- 

 pressed. — It must be carefully observed that the demonstration 

 of the dual nature of inheritance afforded by the facts of amphi- 

 mixis does not necessarily imply that the dual nature of the 

 inheritance will be patent in the full-grown offspring. The 

 offspring is often like both its parents, often particularly like 

 one, often not very like either. The parent of children, the 

 breeder of animals, or the cultivator of plants, has often occasion 

 to remark in the offspring what looks like an entire absence of 

 the characteristics of one of the parents. The foal may seem 

 to take entirely after the sire, as if the maternal inheritance 

 counted for nothing. It is likely that this so-called " exclusive " 

 or "unilateral" inheritance is often more apparent than real, 

 our observation being arrested and preoccupied by a few out- 

 standing features. The certain fact that the resemblance, 

 apparently absent, often reappears in the next generation, 

 shows that the incompleteness was not in these cases in the 

 inheritance, but simply in its expression. We shall return to 

 this subject in connection with the different modes of inheritance. 



4. Each Germ-cell has a Complete Equipment of Heredi- 

 tary Qualities. — It is usually assumed that each of the two 

 sex-cells which unite in fertilisation has in it the potentiality of 

 an organism with a full equipment of the essential characters of 

 the species ; but since the spermatozoon always dies unless it 

 enters the ovum, it is difficult to give experimental proof of the 

 assumption. Some recent daring experiments, which demand 

 confirmation, are very suggestive in this connection. 



Prof. Yves Delage (1898) divided the minute egg of the sea- 

 urchin under the microscope into two parts, one containing the 



