Section 3. 

 Heredity. 



As no living thing arises spontaneously, but is built up from a 

 pair of other living things (with certain exceptions in the lower 

 creation) of the same species, it is probable that so long as man has 

 been able to reason he has regarded the offspring of a given union 

 as consisting of a mixture of its parents. As knowledge became 

 extended, more especially since the origin of species has been traced 

 almost step by step from the lowest to the highest, and shown to 

 result from a common primitive form, the facts underlying these 

 extraordinary changes have been the subject of inquiry, in order to 

 determine the natural laws by which they are brought about. 



The Darwinian Theory of the origin of species was based on the 

 struggle for existence which arises wherever the capacity for repro- 

 duction is greatly in excess of the available food-supply. In the 

 acute struggle the fittest must necessarily survive ; consequently 

 this is determined by heredity. The transmission of favourable 

 variations from parent to offspring must also in the subsequent 

 stress afford those so endowed with greater powers of resistance, 

 and consequently with a better chance of survival. The struggle 

 for existence works in conjunction with natural selection. Darwin 

 laid great stress on the gradual accumulation of small differences 

 in the process of evolution, and he did not believe it possible for 

 Nature to make 'jumps.' Extended experiments have shown that 

 'jumps' are occasionally made, and, moreover, such 'jumps' are 

 heritable. The transmission of acquired characters was regarded 

 by Darwin as a factor in evolution. We shall see presently that the 

 consensus of present-day opinion is against the transmission of 

 acquired characters, though those which are innate, and therefore 

 born with the individual, are of course transmissible. 



What has been stated of the impregnation of the ovum (p. 703) is 

 sufficient evidence that both parents contribute in an equal degree to 

 the construction of the offspring, and there is not much difficulty in 

 the imagination conceiving that not only are the parents concerned, 

 but their ancestors are represented, though in an ever-diminishing 

 degree. An animal or a plant is therefore a mixture of its ancestors, but 

 whether in this complex the individual characteristics remain 

 distinct, or whether they are blended, is the question which at the 

 present day is receiving the closest attention. If they are blended, 

 fresh characteristics may in consequence arise ; if they are not 

 blended, nothing can arise which was not conveyed to the new 

 individual through the generative cells of its parents. 



Galtonism. — That an animal is a mixture of its parents and their 

 progenitors represents a bare statement of the law of heredity with 

 which the name of Galton will ever be associated. As the result 

 of his careful inquiries and elaborate statistical work on man and 

 animals, Galton formulated a law known as the law of ancestral 

 inheritance, which he stated as follows : ' The two parents between 

 them contribute on the average one-half of each inherited quality, 

 each of them contributing one quarter. The four grandparents 



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