460 AN INTRODUCTION TO ZOOLOGY 



The line from the base to the highest point is termed the mode, 

 or in other words the mode is the point at which the largest number 

 of individuals occur, and often the curve balances on each side. 

 Sometimes the curve is asymmetrical because the variation is more 

 limited on one side than the other. This is termed a skew curve. 



Heredity. Heredity is the second great factor. At the com- 

 mencement of the life of one of the higher animals we have the 

 fertilised ovum. This is derived from the union of the sperm, a part 

 of the male parent, and the ovum, a part of the female parent. As we 

 have seen, each parent contributes an equal number of chromosomes 

 to the nucleus of the new organism and these it is that are the impor- 

 tant structures in controlling the course of development. Because 

 of this relation to its parents the offspring resembles them, and 

 we give the name " heredity " to the genetic relation between parents 

 and offspring. The inheritance of an animal, then, is all that it 

 possesses in virtue of its relation to its parents. This relationship 

 has long been recognised and is expressed in a number of popular 

 sayings, such as " like begets like," " a chip of the old block." That 

 the heritage or inheritance is also generally recognised as important 

 is expressed by the old adage, " You cannot make a silk purse out 

 of a sow's ear." The power of handing on characteristics is one of 

 the necessary conditions of evolution ; without it evolution could 

 not have taken place. 



Francis Galton hi 1897 formulated what is known as " Gait on 's 

 law of ancestral inheritance." He says, " the two parents con- 

 tribute between them on the average J('5) of the total heritage of 

 the offspring ; the four grandparents ('25) ; the eight great-grand- 

 parents ('125) ; and so on. Thus the sum of the ancestral contribu- 

 tions is expressed by the series '5, ('5) 2 , (*5) 3 , which being equal 

 to one accounts for the whole heritage." Professor Pearson's work 

 leads him to conclude that the series would be more accurately 

 expressed by '6244, "1988, '0630, etc., but this does not affect the 

 validity of the law ; it only indicates that the actual contribution of 

 the immediate parents is somewhat higher than Galton estimated. 

 It should be noted that Gait on 's law is a statistical law giving the 

 average result of the analysis of large groups, and does not neces- 

 sarily apply to the inheritance in any one individual case. 



, Selection. We have seen what is meant by Variation and 

 Heredity, and now pass on to consider the third factor, Selection, 

 to the action of which the other two are necessary conditions. This 

 is also termed the Struggle for Existence, and was arrived at firstly 

 by a consideration of the manner in which animals multiply. 



Darwin's own classical example is that of the Elephant, one of 

 the slowest breeding of all animals. We may assume as fairly 



