[macallum] SEMI-CENTENNIAL OF THE ORIGIN OF SPECIES 185 



do not obtain. There followed from this the conclusion that the ger- 

 minal cells are unaltered during the life of the individual organism. 



'This conclusion is embodied in the theory formulated by Weismann 

 known as the Continuity of the Germ-Plasm. According to this the sub- 

 stance of the germinal cells which transmits parental characters is not 

 affected during the life of the parent and this substance which Weismann 

 called the germ-plasm is conveyed unchanged from one generation to 

 another. This germ-plasm obtains in every ovum and spermatic element 

 and when the former is fertilized it receives the germ-plasm of the male 

 element. In the development which follows a portion of the blended 

 germ-plasm derived thus from the two parents remains unchanged and 

 is stored up in the germ cells of the embryo. When the latter becomes 

 adult the germ-plasm thus separated and husbanded is conveyed to its 

 offspring. In this way, according to Weismann, the fertilized o^^lm of 



the tenth generation would contain ^^ of the germ-plasm of the first 



but this he holds as sufficient to account for the appearance of remote 

 ancestral characters. 



The germ-plasm so continued from generation to generation is the 

 dominant, the all-determining factor in heredity. It " not only contains 

 the whole of the quantitative and qualitative characters of the species, 

 but also all individual variations as far as these are hereditary." It is, 

 accordingly, " a substance of extreme stability, for it absorbs nourish- 

 ment and grows enormously without the least change in its molecular 

 structure." It is not a simple substance but it " possesses a very com- 

 plex constitution, for it consists of a number of ancestral germ-plasms 

 represented in very different proportions." " Certain ancestral germ- 

 plasms will meet and together produce a double effect; other opposed 

 germ-plasms will neutralize each other; and between these two extremes 

 all intermediate conditions will occur." ^ 



Such was the germ-plasm as Weismann first defined it. It was ab- 

 solutely stable and perpetually immutable. In later years he modified 

 his conception of its characters and he now attributes to it the capacity 

 to change its qualities, which capacity is influenced by marked fluctua- 

 tions in the nutritive stream directed towards the germinal cells. These 

 fluctuations are in their turn due to changes in the environment, as when, 

 for instance, the species is brought into new conditions of existence, for 

 then the nutritive currents within the germ-plasm alter and in conse- 

 quence its qualitative composition varies. These variations may go on 

 indefinitely, and indeed Weismann assumes " that the phyletic evolution 



1 The quotations are from Weismann's " Essays on Heredity," English 

 Translation, The Clarendon Press, Oxford, 1889. 



