CHAPTER I 



why: like begets like 



Many attempts have been made to discover the under- 

 lying reason for tlie fact that hke tends to beget hke. 

 The theories that have been proposed from time to 

 time are both numerous and varied. 



Let us first consider briefly what is known as to the 

 actual mechanism by which an organism is produced 

 by its parents. Let us see what is the actual physical 

 connection between parent and offspring. 



In sexual reproduction, wliich is almost universal 

 amongst the higher animals and plants, the union of 

 two elements is necessary before a new organism can 

 be produced. Li animals these elements are known as 

 the ovum or egg, produced by the female, and the 

 sperm, produced by the male. The sperm cell is an 

 extraordinarily minute body ; the actual size varies 

 greatly, but in many animals it is less than a thousandth 

 of an inch in breadth. The sperm is usually a motile, 

 free swimming body, for aU the world hke an inde- 

 pendent little animal. A common shape is that shown 

 in Fig. 1, where the sperm is seen to have a round 

 " head " and a long, wiiip-Mkc " tail," the only use of 

 which is for locomotion. Sperms are usually produced 

 by the male animal literally in milhons. 



The ovum is usually very much larger than the 

 sperm. It is often microscopic, but in birds, where the 

 yolk is the actual ovum, it reaches a very considerable 

 size. The large size of the ovum, however, is due to 

 the fact that it has been stuffed with food for the use 

 of the young animal. There is every reason to beheve 

 that the essential part of the egg is contained in the 

 nucleus or germinal vesicle, wliich bears a very small 



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