PART II. 

 THE CAUSES OF VAKIATIOK 



CHAPTEE IV. 



BLASTOGENIC VARIATIONS. 



The ultimate cause of blastogenic variationEffect of staleness and 

 of comparative maturity of sex-cells on the characters of organisms 

 Amphimixis Identical twins Transplantation of ova in the 

 rabbit Law of Ancestral Heredity in man and in the Basset 

 hound Regression towards mediocrity Exclusive inheritance. 

 Homotyposis. 



AKGUING from his theory of the continuity of the 

 germ-plasm, first suggested in 1883,* Weismann came 

 to the conclusion that acquired characters were not 

 transmissible. Such acquired characters are due to the 

 direct influence of the environment upon the body 

 tissues of an organism, or are variations of somatogenic 

 origin. Opposed to these are variations due directly to 

 certain peculiarities of the germ-plasm 2 or variations of 

 blastoger^ic origin, which are, on the contrary, hereditary 

 or transmissible. Thus, according to Weismann, all 

 variations are, in respect of their origin, sharply divisi- 

 ble into these two groups, whilst in respect of trans- 

 missibility they are equally distinct. In the present 

 *Vide " Essays on Heredity," Oxford, 1889, p. 71. 



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